[paulbalandan]

Description
Part 1 of the user guide migration agreed in #10683. This PR only adds docs/, the user guide converted from user_guide_src/source with codeigniter4/docs-update. Nothing builds or deploys it yet, and the RST guide is unchanged.

The user guide is frozen from this PR until the switch (PR 4 below) is merged. Please do not merge changes to user_guide_src in the meantime.

If you have an open PR that changes RST, convert its pages after the cutover with docs-update (make convert PAGE=libraries/email) and apply your edits to the Markdown in docs/.

Planned PRs:

1. This PR: the Markdown pages.
2. The MkDocs build and CI check.
3. The release tooling ported to Markdown.
4. The switch: removes user_guide_src, changes the deployment and adds the codeigniter4/userguide package files.
5. Contributor docs and tooling references.

The converter's checks compare the RST and the Markdown on every run. All pass for this branch: 325 pages, 485 PHP signatures, 1,958 code samples, 841 admonitions and no leftover RST markup. The strict MkDocs build is clean.

The pages are generated, so a review of docs/index.md and a few pages is enough.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide