docs: Minor improvements to contributing guide

[rodrigogiraoserrao]

This PR makes some minor improvements to the contributing guide, reflecting the experience I just had setting everything up.

• Add two notes to things that are required when setting up the dev environment for the first time and when building the docs for the first time.
• Fix formatting of a nested list that was being poorly formatted by dprint because it uses a different standard from the one followed by the markdown parser used by MkDocs.
• Add a “copy to clipboard” button to all code snippets which is SUCH a quality of life improvement!

[ritchie46]

Fix formatting of a nested list that was being poorly formatted by dprint because it uses a different standard from the one followed by the markdown parser used by MkDocs.

Will a next dprint call undo this again?

[ritchie46]

Add a “copy to clipboard” button to all code snippets which is SUCH a quality of life improvement!

<3

[rodrigogiraoserrao]

Will a next dprint call undo this again?

Nope, because I used the <!-- dprint-ignore-start --> directive to prevent dprint from formatting that nested list.