Rule 16 uncaptured cases and false positives

Please see comments from @ru-fu in #52, an I've added a few additional cases.

Cases include:

- [ ] - Capturing all MyST directives that contain `#`
- [ ] - Not capturing code blocks declared by indentation (MD)
- [x] - Not capturing code blocks delimited by `::` (RST)
- [ ] - Not capturing indented code blocks, as the initial delimiter has a start of new line requirement (MD)
- [ ] - Will capture content with `#` contained inline (e.g. URLs with anchors or other random strings)
- [x] - Not capturing MyST colon fenced code blocks
- [ ] - Unable to parse included code files, or excerpts from code - likely out of scope, would require an additional [scoping value](https://vale.sh/docs/topics/scoping/) and additional rules.

Start by reading the comments from @ru-fu in issue #52 and the additional cases listed here. The payload names no files, tests, or entry points, so locate the implementation of Rule 16 and its existing checks first. Done means addressing the unchecked capture cases without regressing the checked exclusions.

Written by the indexing model from the issue text.