Add # links for headings in generated Roc documentation

[lukewilliamboswell]

This issue proposes that we add support for markdown # links. In a URL, a hash # points a browser to a specific spot in a page or website.

The headings generated from doc comments in https://www.roc-lang.org/builtins/Str are not able to be linked to as they are just simple <h2> tags.

For example the following doc comments would generate a link for the Syntax heading.

## ## [Syntax](#syntax) {#stynax}
##
## The most common way to represent strings is using quotation marks:
##
## ```
## "Hello, World!"
## ```

[kencruz]

Hi, I'd like to work on this one please