Update the "Community" link(s) in the ToC panel

[TibsAtWork]

We now have a Community page at aiven.io (http://avien.io/community) and should reflect this in the documentation.

One possibility is to replace the current "Community" link with a direct link to that page,
and then move the existing ToC entries into a "How to Contribute" clause. The two challenges could go under a new "Challenges" item.

We should consider trying out some layouts to see what works.

[angelinekwan]

@TibsAtWork Is this something we can try in a PR already or ...

[TibsAtWork]

@angelinekwan Yes, definitely - a new branch to try what it looks like and a corresponding PR is the right way to go.
It probably won't be me in the near term, but someone else may pick it up.

[harshini-rangaswamy]

FYI @TibsAtWork - This PR is in progress to include the Community information in the docs.