`RDoc::Parser::ChangeLog` doesn't handle keep-a-changelog style CHANGELOG correctly

Background

This is not a "bug", because it says

This parser is meant to parse the MRI ChangeLog, but can be used to parse any # {GNU style ChangeLog}[http://www.gnu.org/prep/standards/html_node/Style-of-Change-Logs.html].

However, when we run rdoc command, it automatically parses CHANGELOG.md file and outputs some wrong results. This is done by default so it's not a desired behavior.

Problem

When parsing a markdown snippet like below:

## [3.0.0] 2023-10-11

### IMPORTANT

**This release contains an important bug fix that can cause data corruption.**
**If you're using Ruby 3, it's highly recommended to upgrade to [v3.0.0](https://rubygems.org/gems/alba/versions/3.0.0)**
**If you're using Ruby 2, please upgrade to [v2.4.2](https://rubygems.org/gems/alba/versions/2.4.2) that contains bug fix only as soon as possible.**

it should create a link for v3.0.0 since it's a valid markdown syntax (GitHub renders it as a link, for example, see https://github.com/okuramasafumi/alba/blob/main/CHANGELOG.md#300-2023-10-11).

However, the resulted HTML is:

<h2 id="label-5B3.0.0-5D+2023-10-11">[3.0.0] 2023-10-11<span><a href="#label-5B3.0.0-5D+2023-10-11">&para;</a> <a href="#top">&uarr;</a></span></h2>

<h3 id="label-IMPORTANT">IMPORTANT<span><a href="#label-IMPORTANT">&para;</a> <a href="#top">&uarr;</a></span></h3>

<p><strong>This release contains an important bug fix that can cause data corruption.</strong> <strong>If you’re using Ruby 3, it’s highly recommended to upgrade to {v3.0.0}[https://rubygems.org/gems/alba/versions/3.0.0]</strong> <strong>If you’re using Ruby 2, please upgrade to {v2.4.2}[https://rubygems.org/gems/alba/versions/2.4.2] that contains bug fix only as soon as possible.</strong></p>

The link parts are not rendered correctly.

Solution

The comment in this parser says it's for GNU style changelog only, but I believe most projects use keep a changelog style. They don't look compatible.
So we have two ways. One is to improve the current changelog parser so that it can parse both styles. Another is to create another parser for keep-a-changelog style specific.

We can also discuss if including changelog into a resulted documents is desired or not. YARD doesn't include it, so maybe nobody wants it.

Start at RDoc::Parser::ChangeLog and reproduce the rdoc command behavior with the provided CHANGELOG.md keep-a-changelog snippet. Compare the current GNU-style parsing with the expected Markdown links and determine whether one parser or a separate parser should support both styles. Done means links such as v3.0.0 render correctly and the desired inclusion of changelogs in generated documents is settled.