Add snippet compiler docs

[pikinier20]

This PR is related to #12987

closes#12843

[julienrf]

Thank you for taking the time to write some documentation!

Besides the specific comments I left, I have the following general recommendations. The reference documentation should first and foremost answer the following questions: what can I do? And how do I do it? Other questions such as “how does that work under the hood” are secondary and should come after the main content, IMHO. Also, I don’t think we should give too much background explanation about why and how you introduced this or that feature (that content would be great for a blog post, though, or for the scala3/scaladoc.md page). I mean, it’s always good to first show some motivating example/content before showing a feature, but we don’t necessarily want to hear all the story behind. I hope this helps :)

Do you mind attaching a screenshot of the rendered documentation? (the page snippet-compiler.html)

[romanowski]

Looks nice!

[romanowski]

cc @alvinj

[alvinj]

I added a number of suggestions here, and I hope they’re hopeful. If you have any questions on any of my comments, please let me know. (Also, if I did anything wrong, please let me know that as well!) Thanks!