docs: collapse archived user guide versions behind a single Older Versions page

[andygrove]

Which issue does this PR close?

Closes #4425.

Rationale for this change

The user guide sidebar listed all five published versions inline (one snapshot plus four release branches). For most readers that list is noise: nearly everyone wants the latest stable release, the development snapshot is occasionally useful, and the archived versions are rarely consulted. The issue describes the list as "quite annoying" and asks for the current stable to be the default, with the snapshot and older versions tucked away.

A pydata-sphinx-theme version dropdown was considered but rejected: it assumes one Sphinx build per version at distinct URL prefixes, while Comet ships a single combined build with versions in subdirectories. Adopting the dropdown without restructuring the build would be cosmetic only and would not remove the toctree clutter. Restructuring to one build per version is a separate, larger project.

What changes are included in this PR?

• Replace the five-entry user guide toctree in docs/source/user-guide/index.md with three entries: 0.16.x (current), Development Snapshot, and Older Versions. Refresh the body copy to point readers at the current stable as the default.
• Add docs/source/user-guide/older-versions.md, a small page whose own toctree registers the archived versions (0.15.x, 0.14.x, 0.13.x). This keeps Sphinx warning-free while collapsing the archives behind a single sidebar entry.

No changes to generate-versions.py or the build script: the existing release-branch cloning and latest_released_version / previous_versions split already match the "one current, rest archived" model, and the new index just consumes those generated directories.

Sidebar before vs after, under "User Guide":

Before:                          After:
0.17.0-SNAPSHOT                  0.16.x (current)
0.16.x                           Development Snapshot
0.15.x                           Older Versions
0.14.x
0.13.x

How are these changes tested?

Verified locally by running sphinx-build against a copy of docs/source with mock versioned directories. The build succeeds with no structural warnings, the User Guide sidebar shows exactly the three expected entries, and the Older Versions page lists working links to 0.15.x / 0.14.x / 0.13.x. CI will exercise the full docs/build.sh path.