docs: one short line per change in the changelog (#79)

Makes the changelog readable at a glance: every change is one short
sentence, followed by links to the rule, the guide and the PR. The page
goes from about 6,400 words to about 2,300 (most of what is left is
links).

## Before vs after (v2.20.x)

| Before | After |
|---|---|
|
![before](https://github.com/commit-check/commit-check.com/raw/6804862e42d1e7744dda505d9bd5fb27ffec874a/.pr-shots/changelog-before.png)
|
![after](https://github.com/commit-check/commit-check.com/raw/6804862e42d1e7744dda505d9bd5fb27ffec874a/.pr-shots/changelog-after.png)
|

Example, v2.20.2:

- Before: "`--ai-only` is now `--ai-attribution-only` — the new name
says what the run is limited to: the AI attribution of the text, and it
pairs with the `--ai-attribution` policy option. `--ai-only` still
works, so a script written against 2.20.1 needs no change; it is no
longer listed in `--help`. See Checking a pull request description. See
PR 677."
- After: "`--ai-only` is renamed `--ai-attribution-only`; the old name
still works. (Guide, PR 677)"

## What changes

| Page | Change |
|---|---|
| Changelog | One sentence per change, v2.20.2 back to v2.0.0. Anchors
(`#v2202` etc.) unchanged, so blog links still work. Section names
unified to Breaking / Added / Changed / Fixed. Internal-only items
(refactors, a test mock fix, SonarQube, OpenSSF badge) dropped; the
GitHub releases still list them. Highlights rows for 2.20.1, 2.20.0,
2.19.2 shortened. |
| Changelog, v2.6.0 | Now lists `--format json` and the Python API
(commit-check/commit-check#396), which shipped in 2.6.0 but were missing
from the entry. Added CLI PR links where entries had none
(commit-check/commit-check#412, commit-check/commit-check#544,
commit-check/commit-check#559, commit-check/commit-check#560). |
| Homepage FAQ | "Python 3.10 through 3.15 … eighteen combinations" →
"3.11 through 3.15 … fifteen combinations" (3.10 dropped in v2.20.0; CI
matrix is 3.11–3.15). |
| Rules, CC013 | The 120-word "What it does" sentence is now a short
bulleted list. Same content. |

## Checks

- `mkdocs build --strict`: clean.
- `python -m pytest tests -q` with commit-check 2.20.2: 18 passed.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Documentation**
* Updated release highlights to one-line summaries, ordered newest
first, with links to full release notes. Detailed entries are more
concise.
* Revised Python support guidance to cover versions 3.11–3.15 and 15 CI
wheel-install combinations.
* Clarified the documented AI attribution formats recognized by CC013,
including Copilot coding-agent co-author lines.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Xianpeng Shen <3353385+shenxianpeng@users.noreply.github.com>

Author: shenxianpeng