docs: add release notes for 1.1.0 with known regressions and workarounds

[andygrove]

Which issue does this PR close?

None. It comes out of the 1.1.0 regression audit (#6399), and the regressions it lists are tracked in #6402.

Rationale for this change

Comet publishes a change log for each release, but no release notes. The upgrade guide already says that correctness fixes "appear in the release notes", and the audit of 1.1.0 found regressions that users should know about before they upgrade, along with the settings that avoid them.

What changes are included in this PR?

• A new top-level Release Notes section in docs/source/release-notes/, with an index page and a page for 1.1.0. It sits before the change log in the site navigation.
• The 1.1.0 page opens with how many 1.0.0 bugs the release fixes, then lists the known regressions: two wrong results and two errors. Each entry links its issue, says how to avoid it, usually with a setting, and says what else that setting sends to Spark. A last entry covers fix: revert unsafe partial aggregates after final fallback #5421, which moves some partial aggregates back to Spark when the Comet shuffle manager isn't installed. That one is a deliberate trade for correctness rather than a regression, but it can make those jobs slower.
• The upgrade guide's 1.1.0 section links to the release notes, and gains a subsection on native Iceberg reads with IRSA. Since fix: Native S3 scan on EKS/IRSA turns a transient STS throttle into a hard 403 storm #6025, they no longer fall back to the node role when the web-identity call fails. The subsection names the per-catalog opt-out, and the setting to use for tables loaded by path.

1.1.0 will be released from an rc2 cut from branch-1.1, which has the fixes for #6424 and #6464 (#6449), #6423 (#6489), #6426 (#6486), #5507 (#6487), #6505 (#6511), #6334 (#6491), #6466 (#6488), #6506 (#6540) and #6425 (#6554), so those aren't listed. #6543 fixes #6504 and #6546 on main, and if its backport merges before rc2, those two entries come out as well. #6546 turned up after the audit, while fixing #6504.

The bug counts come from the audit's data. rc1 has 131 fix: pull requests. 91 of them fix bugs that shipped in 1.0.0, and about 40 of those 91 fixed results that differed from Spark.

How are these changes tested?

Each workaround was run against its regression's reproducer on a 1.1.0-rc1 build. Without the setting the reproducer still differs from Spark, and with it the result matches Spark. The Iceberg entries were checked with Iceberg 1.11.0.

Prettier passes on the changed files. I couldn't build the Sphinx site locally, and the docs job only runs on pushes to main, so the new section hasn't been rendered yet.