build(docs): update Sphinx Stack to 2.0

[jahn-junior]

• Update docs tooling to Sphinx Stack 2.0 (see full diff).
• Replace any other references to the 'Starter Pack'
• Lock dependencies
• Fix the autobuild loop by ignoring symlinks and generated dirs with SPHINX_AUTOBUILD_OPTS

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• I've followed the contribution guidelines.
• I've signed the CLA.
  I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
  In documents I changed, I added a meta description if one was missing.
  I've updated the relevant release notes.

[Copilot]

Pull request overview

This PR updates the project’s documentation tooling/configuration to align with Canonical’s Sphinx Stack 2.0, including refreshed dependency sets and updated docs build/lint workflows.

Changes:

• Updated Python dependency groups / lockfile to match Sphinx Stack 2.0 and related extensions.
• Refactored docs build tooling (docs Makefile, _dev tooling, ignore patterns) and refreshed docs config (conf.py).
• Replaced “Starter Pack” references with “Sphinx Stack” across docs.

Reviewed changes

Copilot reviewed 16 out of 20 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
uv.lock	Locks new/updated docs toolchain dependencies required by Sphinx Stack 2.0.
pyproject.toml	Renames/updates docs dependency group and aligns specifiers with Sphinx Stack 2.0.
docs/requirements.txt	Updates docs subproject requirements to Sphinx Stack 2.0 extension set/versions.
docs/Makefile	Updates docs dev/build targets and introduces SPHINX_AUTOBUILD_OPTS for autobuild behavior.
docs/conf.py	Updates Sphinx configuration for Sphinx Stack 2.0 and enables new extensions/settings.
docs/_templates/header.html / docs/_templates/footer.html	Adds theme template overrides for header/footer content.
docs/_dev/*	Introduces/updates Sphinx Stack _dev tooling/config (Vale, pa11y, updater, version marker).
docs/README.md / docs/explanation/documentation.rst	Updates documentation links and migration instructions to Sphinx Stack.
docs/.gitignore	Adjusts ignored paths for new .venv and _dev layout.
common.mk	Updates wording to reflect Sphinx Stack naming.

Comments suppressed due to low confidence (1)

docs/_dev/update_sp.py:25

• SPHINX_UPDATE_DIR references DEV_DIR, but DEV_DIR is not defined anywhere in this script, so running the updater will raise NameError before doing any work. This should point at the local _dev directory (SPHINX_DIR).

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[bepri]

Below are comments that probably belong in the Sphinx Stack repository instead. Let me know if you want me to make these into issues over there.

[medubelko]

The main commands tested fine locally. I added meta descriptions to all of the major indexes, because the link summaries in llms.txt were being truncated.

As an aside, it seems sphinx-llm can't process wrapped lines in .rst files, which is another reason to seriously consider moving away from wrapping.