chore(docs): Update sitemap and intersphinx

[erinecon]

What this PR does

Updates the sitemap configuration.
Updates outdated or removed intersphinx entries.

Why we need it

The RTD project began failing due to the broken intersphinx entries, e.g., https://app.readthedocs.com/projects/canonical-synapse-juju-charm/builds/4225293/

Note that, since the broken intersphinx entries are in track/1, it is expected that the "check URLs" workflow will fail until we merge the fix.

Update the sitemap configuration to be consistent with our other public documentation sets, and to avoid issues when we introduce versioning. Validate with https://canonical-synapse-juju-charm--1206.com.readthedocs.build/1206/doc-sitemap.xml

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• [N/A] I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• [N/A] I added or updated tests as needed (unit and integration)
• [N/A] If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• [N/A] If this is a Grafana dashboard: I added a screenshot of the dashboard
• [N/A] If this is Terraform: terraform fmt passes and tflint reports no errors
• [N/A] If this is Rockcraft: I updated the version