docs(swiftpm): rename __doc__ to __docs__, drop two superseded docs, correct the rest

[chrfalch]

Rebased onto main after #57757, #57756, #57762 and #57744 landed. Because this
PR renames __doc__ and Prettier-formats every file, a hunk-level rebase was the
wrong tool — it stopped on the second of nine commits and the reformat commit then
conflicted with everything after it. So the history is replayed on top of the
new main instead, and reordered to put Prettier before the content edits, which
makes the substantive diff reviewable rather than tangled with reflow.

Verified nothing those four PRs added was lost: every heading on main survives
except the two this PR deliberately renames, and their facts — the
artifactsVersionOverride pin, the autolinking-config-command pin, the
pre-injection build-setting values, and the promoted-scalar caveat — are carried
into the new "Files the tool touches" table rather than sitting in a row this PR
deletes.

Summary:

Housekeeping and accuracy pass over packages/react-native/scripts/spm/__docs__.
Docs only — no code or test changes.

Rename. scripts/spm/__doc__ was the only __doc__ directory in the repo;
every other subsystem uses __docs__ (27+ directories, and the convention
documented in __docs__/GUIDELINES.md). The misspelling also meant these files
were published to npm: package.json includes scripts/spm in files and
excludes !**/__docs__/**, which the misspelled directory dodged.

Removed two superseded documents. rfc-spm-xcframework.md (707 lines) was a
stale ancestor of RFC0994, still describing spm init, xcodeproj generation, the
.xcodeproj.legacy rename migration, a spm clean command that does not exist,
stub Package.swift files and VFS overlays. spm-plugins-assessment.md evaluated
SwiftPM plugins as a replacement for the injected build phases — conclusion valid,
detail stale (it listed a "Prepare VFS Overlay" phase). Both have had their durable
content folded into RFC0994.

Corrected the remaining docs against the implementation. The substantive one:
auto-sync is two hooks, not one — a scheme pre-action in the app's shared
scheme plus the build phase. The docs described only the phase.

Two claims here are corrections of my own earlier drafts, made after testing them
on private/helloworld rather than reading the code:

• Neither hook can bootstrap a clean checkout. With build/ deleted,
  xcodebuild -scheme … build fails in nine lines of log, Resolve Package Graph
  first, the pre-action never running. The one-time setup run really is required,
  and the ordering table now shows the measured sequence.
• A sync failure is not unconditionally non-fatal. Exit 2 — a dependency with no
  Package.swift — maps to exit 1 and fails the build deliberately; only other
  non-zero codes warn. As written, the doc also contradicted the exit-2 behaviour
  documented elsewhere in the same file.

Smaller fixes: hermesvm → hermes-engine (that name is in no code), plugin
watchPaths added to the staleness inputs, and the #auto-sync-build-phase
anchors left dangling by the heading rename.

Added a __docs__/README.md index, per GUIDELINES.md, plus a "Files the
tool touches" table — there was no single answer to what the tool creates or
modifies. Writing it surfaced that add creates or appends to ios/.gitignore
(documented nowhere), and that deinit does not revert that block, anything
--deintegrate changed, or a promoted array setting — so the "exact inverse of
add" claim needed qualifying in three places.

Documented spm.dependencies, the SwiftPM analog of a podspec's
s.dependency, which was implemented but absent from every doc, and corrected the
scaffold prerequisite: it is add/update that stops, with a distinct exit code 2,
not the build.

Prettier-formatted the docs, in its own commit. These were the only unformatted
markdown files in the repository, so prettier --list-different "./**/*.md" goes
from three failures to clean.

Changelog:

[Internal] - SwiftPM docs: rename __doc__ to __docs__, remove two superseded
documents, and correct the rest against the implementation

Test Plan:

• npx prettier --check "packages/react-native/scripts/spm/__docs__/*.md" passes;
  npx prettier --list-different "./**/*.md" is empty for the whole repo.
• All 19 intra-doc anchor links verified against GitHub's slug rules; none dead.
  Every relative path out of the new README resolves, including the root-index hop.
• npm pack --dry-run --ignore-scripts lists no scripts/spm/__docs__/ entries
  while every scripts/spm/*.js still ships.
• The Prettier commit is content-neutral: comparing [A-Za-z0-9]+ token streams
  across it, all three files are identical word for word.
• Behavioural claims traced to source: VALID_ACTIONS in setup-apple-spm.js;
  injectOrCreateScheme / addPreActionToScheme and the RC -eq 2 branch of
  buildSyncAutolinkingScript in generate-spm-xcodeproj.js; BUILTIN_FRAMEWORKS
  in flavored-frameworks.js; generateXCFrameworksPackageSwift in
  generate-spm-package.js; ensureGitignoreSpmEntries and its action === 'add'
  guard; expand-spm-dependencies.js; reportMissingManifests.
• The fresh-clone and ordering claims were measured on private/helloworld with
  Xcode 26.6, both artifact flavors cached, and the app restored afterwards.

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D116598871.