Add fnm as an option for Node version manager

[ayuday]

add fnm for Node version manager

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[MattieTK]

TIL Volta is unmaintained, fnm has 2x as many stars and a more recent update.

We should probably also update to suggest https://mise.jdx.dev/ which is Volta's suggested replacement pathway. @ayuday do you want to update this to also include that?