feat: JAX default install, [jax] alias + install docs (#702)

[Jammy2211]

Summary

Closes the packaging half of #702: JAX is now a default dependency of the stack. autolens receives jax, optax and jax_zero_contour transitively through the autogalaxy → autofit → autonerves base-dependency chain (each promoted in its own repo's sibling PR), so the changes here are the [jax] extra becoming a declared no-op alias (#687), the floor comment recording the follow-up floor bump, and the install docs.

Docs (docs/installation/pip.md, conda.md): pip install autolens is now the JAX-enabled command; the [jax] form is documented as a still-working alias; Intel Macs (no jaxlib macosx_x86_64 wheels ≥0.7) are documented as automatically falling back to the fully supported NumPy-only path, with a loud import-time warning.

API Changes

None — packaging metadata and install docs only.
See full details below.

Test Plan

• pyproject parses; [jax] alias resolves
• Clean-venv install of the promoted chain pulls jax 0.11.1 + jaxnnls automatically
• No-JAX CI leg (PyAutoHeart lib-tests.yml unittest-nojax) green once the Heart PR merges

Full API Changes (for automation & release notes)

Changed Behaviour

• pip install autolens now installs JAX by default via the family chain (except Intel macOS, which falls back to NumPy-only with an import-time warning).

Migration

• Before: pip install autolens[jax]
• After: pip install autolens (the [jax] form still works — no-op alias)

Sibling PRs: PyAutoNerves, PyAutoFit, PyAutoArray, PyAutoGalaxy (packaging), PyAutoHeart (no-JAX CI leg). Follow-up after the first promoted release: bump the intra-family floors to it.

Generated by the PyAutoLabs agent workflow.

[Jammy2211]

Workspace PRs: PyAutoLabs/autolens_workspace#486, PyAutoLabs/HowToLens#71