docs: Update to the latest version of the Sphinx Stack

[skoech]

Description

This PR updates the MicroCeph docs to the latest version of the Sphinx Stack (v2.0). It brings in updated tooling, and adds missing CI checks.

• Fixes CI: Sphinx build errors from unsupported pyspelling module #678
• Potentially fixes domain migration issue: (related to feat: Configure docs for migration to canonical.com (add migration branch for main) #752)

Key changes

• Syncs conf.py with the latest Sphinx Stack template
• Syncs GitHub workflows with Sphinx Stack (cla-check.yml, check-removed-urls.yml)
• Replaces bundled extension dependencies with individual packages in requirements.txt
• Adds redirects.txt since Sphinx Stack 2.0 deprecates the sphinx-reredirects extension, which defines redirects inline, in favor of sphinx-rerediraffe, which requires redirects to live in a separate redirects.txt file
  • Removes redirects dict from conf.py
• Changes FETCH_SCRIPT variable from SOURCEDIR to DOCS_SOURCEDIR in Makefile
• Adds _dev/ directory with updated tooling (pre-commit hooks, Vale config, pa11y script, pymarkdown) - renamed from _.sphinx to better reflect its purpose (breaking change)
• Removes the reuse/ directory to reduce complexity and improve source readability (breaking change)

Features

• sphinx-llm support: Adds sphinx-llm extension that generates extra files (llms.txt, llms-full.txt, .md copies of each page) alongside regular HTML build, that are optimized for AI tools. This makes the documentation consumable by LLMs and MCP-based tools.
• Dependency constraints: All extensions are now pinned to their major version to prevent upstream breakages from surprise releases. Most extensions now uses the ~= operator (compatible release), which locks to the current major version, which means that we'd still get minor updates (e.g., bug fixes, security fixes and minor improvements/features) without risking a surprise breaking change.
• Copyright statement: Footer now includes license metadata (AGPL-3.0)- added to html_context in conf.py. Sphinx Stack 2.0 aligns the copyright statement with legal standards

Release notes: https://github.com/canonical/sphinx-stack/releases

Type of change

• Documentation update (change to documentation only)
  • Documentation tooling, specifically, not content

How has this been tested?

Tested locally; builds pass with no errors or warnings:

• make run: Builds and serves successfully
• make spelling: No spelling errors; Ceph-specific terms caught added to .custom_wordlist.txt
• make woke: No inclusive language issues
• make linkcheck: No broken links

Contributor checklist

• self-reviewed the code in this PR
• added code comments, particularly in less straightforward areas
• checked and added or updated relevant documentation - N/A
• added or updated HTML meta descriptions for any new or modified documentation pages (see #643) - N/A
• verified that page title and headings accurately represent page content for new or modified documentation pages - N/A
• checked and added or updated relevant release notes
• added tests to verify effectiveness of this change

[lmlg]

lgtm, the tests failures have been explained and are expected.

[skoech]

Thanks, @lmlg. Attempting to fix the check-removed-url check failure.