Fix internal links involving README files resulting in 404 errors

[CCJK123]

Fixes #984.

Did my best to address the concerns mentioned in #1921, by opting to:

• Use the mdbook-markdown/pulldown-cmark and url crates instead of regex to identify and correct the relevant links (comment advising against regex usage)
• Modify the index preprocessor instead of fix_link in mdbook-html (previously adjust_links) to maintain clear separation between the preprocessor and the rest of the codebase (comment mentioning this consideration)

Also duplicated some code from HtmlRenderOptions to get the markdown options for creating a parser via mdbook-markdown, not sure whether this should be abstracted out elsewhere. The testcases I wrote just use the default options.

Hopefully this sufficiently addresses the issue, but do lmk if there's any changes/improvements I shd make!

[CCJK123]

Realised that this approach seems to be lossy due to the current way pulldown-cmark and pulldown-cmark-to-cmark work, and can result in some information being lost, and possibly resulted in the following test failures:

• markdown::basic_markdown: A " gets converted to “, though there are other "s in the same test that aren't converted; disabling smart_punctuation does not resolve the issue
• markdown::definition_lists: <p> tag is not preserved within <dd> fsr
• preprocessor::extension_compatibility and renderer::backends_receive_render_context_via_stdin: Newlines at the end of markdown content get removed (see Empty lines and whitespace are not preserved Byron/pulldown-cmark-to-cmark#94)
• rendering::header_links: Nesting order of <strong> and <em> tags was reversed, inconsequential in this case but not necessarily so in other cases

Perhaps regex is still a viable approach, though it'll need to account for the various types of links. Tolerating some information loss is also an option. Otherwise, the issue will continue to be unresolved due to being blocked by pulldown-cmark/pulldown-cmark-to-cmark.

Would appreciate input on how best to proceed, thanks!