Strip markdown HTML comments within paragraph

[tompng]

Standalone HTML comments is converted to RDoc::Markup::Raw. However, HTML comments in a paragraph were not handled correctly.
Since RDoc paragraph can't contain Raw, inline HTML comments should be stripped. HTML comments in a paragraph "\n<!-- -->\n" will generate double newline "\n\n", so ToHtml also needs to handle this double newline.

Fix this markdown comment rendering

##
# :markup: markdown
# <!-- comment1 -->
# hello
# <!-- comment2 -->
# world

Before

<!-- comment1 -->
<p>hello &lt;!– comment2 –&gt; world</p>

After

<!-- comment1 -->
<p>hello world</p>

[matzbot]

🚀 Preview deployment available at: https://d3cb0ca4.rdoc-6cd.pages.dev (commit: 9037f7b)