Use markdown instead of ocamldoc for docblock comment

[chenglou]

• Safe change. Just comments.
• Odoc throws exception for certain snippets; this causes the editor plugin to fail to display the code block. Now we're just gonna pass through the raw markdown block to be displayed.
• More familiar with the JS audience (and most other folks really).

I've converted over:

• [foo]
• @example [{...}] and miscellaneous blocks
• ul, ol
• {b boldStuff}
• {i italicStuff}
• {%html} blocks
• Things that don't need escape anymore, e.g. \[\@\@module\]
• @return -> **return**. Same for @param, @raise and @since.
• Use markdown link for @see.
• 2 indentations instead of 4, since 4 means a code block in markdown.
• Double newline for a rendered newline (markdown single newline doesn't break line in output).

[chenglou]

I'll squash merge once this is approved, to avoid noisy commits

[chenglou]

Some regex personal notes for posteriority:

([^{])\[([^%|@\\].*?)]

revert:
\[
`.{1,3};.+
`]
\`
`\

check more:
\@
[^\[@]@[^@]       <- check also
`@       <- check also
\{[a-zA-Z]  <- {b Unsafe}, {!head} and others
  like \{b (.+?)\}
@example {[ <- check also
{%html:
@return
@raise
@param
@since
@see
{!Belt.Array}
MDN links