Sdg23 add contribute page

[namiroues]

This PR addresses #441.

[micaela-matta]

thanks! looking good

[orbeckst]

I think this page needs to make clear that there's much more to the MDAnalysis Project than just the core library and the User Guide. Please see comments & please ask here (and in discord) if you want to bounce ideas.

[namiroues]

Thanks @micaela-matta, @orbeckst and @RMeli for your comments. I redesigned the page based on your suggestions.

[namiroues]

Thanks @micaela-matta for your last comments. I've incorporated the changes.

[orbeckst]

The rewrite is really nice.

My remaining concerns are

• We should mention that we have other code repos that are not MDAKits and there contributors are also welcome.
• I am hesitant to include content in this page that needs updating. Someone has to (1) do it regularly (nobody does), (2)has to remember that content needs to be updated here (eg for GSoC, events). And when things are not being updated, the page feels stale sooner than strictly necessary. If there's a way to automate inclusion of content (e.g., based on tags in the meta data) then that would be neat, though.

[namiroues]

Thanks @micaela-matta for your comments. I've updated the page.

[orbeckst]

This is all looking very good. I have a bunch of suggestions and one technical request: use post_url liquid tags when referencing blog posts. This is necessary to generate reliably stable links.

[micaela-matta]

thanks @namiroues - some last minor changes from me

[namiroues]

Thanks @micaela-matta. I've made the changes.