chore: update documentation links for 1.0.0 release

[andygrove]

Which issue does this PR close?

N/A — release chore for 1.0.0.

Rationale for this change

Publishes the 1.0.0 user guide and makes it the default (current stable) version in the
documentation, following the "Publishing Documentation" step of the
release process.
Mirrors the pattern from #4698 (0.17.0) and #4314 (0.16.0).

What changes are included in this PR?

• docs/generate-versions.py: latest_released_version = "1.0.0", and 0.17.0 moves into previous_versions
  (older guides are kept, per docs: restore 0.13 user guide dropped in 0.17 release #4704).
• docs/build.sh: clean up the locally cloned comet-0.17 and comet-1.0 branch dirs before regenerating.
  comet-0.17 was missing because the rm list historically only tracked previous_versions; without a line per
  cloned version a rebuild silently reuses the stale clone from the previous run.
• docs/source/user-guide/index.md: current stable release is now 1.0.x, sidebar entry points at 1.0/index.
• docs/source/user-guide/older-versions.md: point the "current stable release" link at 1.0 and add 0.17.x to the list.
• docs/source/contributor-guide/release_process.md: refresh the "Publishing Documentation" instructions, which
  still referred to docs/source/index.rst and the _toc.user-guide-links-versioned section — neither exists since
  the sidebar overhaul. The section now lists the four files actually involved.

Not included (separate follow-ups per the release process): bringing docs/source/changelog/1.0.0.md into main,
which happens after the vote passes.

How are these changes tested?

branch-1.0 exists and has generated (frozen) user guide content under docs/source/user-guide/latest/, including
index.rst and the per-Spark-version compatibility pages, so the versioned guide will render. The docs build itself
is exercised by the documentation CI workflow.

[mbutrovich]

Thanks @andygrove!