Rewrite docs for Functions

[misteryeo]

• Moved Functions up a level in the hierarchy under Connections and added a few extra sections
• Removed any old legacy functionality references as all of this is no longer relevant
• Included more hyperlinks to other parts of Segment docs as well as more areas asking for users to reach out to support
• Tidied up all the headers to be consistent across Source and Destination Functions and simplified them so it’s easier to navigate

There needs to be some work down for updating visuals + adding in new nicer visuals. Waiting to hear back from Design team on resources here.

For anyone from @segmentio/functions-team reviewing this PR, would love eyes overall but in particular:

[ ] Any content suggestions for the Functions Overview page. I’ve been writing so much about Functions lately, burning out a little bit :lol: The Personas Overview page is a great benchmark or reference for inspo!
[ ] As you’ve all been on-call and answered questions multiple times, is there anything missing from the documentation shared here?
[ ] List of dependencies are complete and 100% correct in terms of versions. Under Source Functions, can I get some help filling out descriptions for the three other dependencies - form-data v2.4.0, oauth v0.9.15 and xml v1.0.1 🙏 I didn’t know we supported the XML one?!
[ ] Overall accuracy of our documentation, I’ve tried my best to verify aspects of these docs but would appreciate everyone’s review here as we head to GA!

[sanscontext]

Hey @vadimdemedes in the interests of not making this branch's commit chain any longer, do you want to just push a commit with all your code changes? If they don't need a second reviewer, I think this is fine.
I also had some notes on product names, and giving more guidance. :)

[vadimdemedes]

in the interests of not making this branch's commit chain any longer, do you want to just push a commit with all your code changes?

Why not? You can combine multiple suggestions into one commit by using "Add suggestion to batch" button.

[sanscontext]

TIL about batching, thanks! @vadimdemedes but do these code changes need a dev review?

[vadimdemedes]

@chriso Could you take a look at code suggestions above?

[sanscontext]

@andy it looks like we have a few outstanding code-format edits from Vadim. I'll review the branch conflicts today.

[sanscontext]

Okay, just ready for any outstanding suggestions from Vadim and any new updates!

[sanscontext]

🚀