Add a "Getting Involved"/"Contributing" page

[ronaldtse]

From @dewyatt :

add a "Getting Involved"/"Contributing" type of page on the website too

Originally from https://github.com/riboseinc/rnp/pull/891

@dewyatt could you help get this page started? I remembered we had an older contribution guide but I couldn't find it... Thanks!

[ni4]

@ronaldtse Contribution guide was migrated here: https://github.com/riboseinc/rnp/blob/master/docs/develop.adoc