Clearly link Contributors pages

Hi,

Attached is a patch to clearly link the three different pages related
to recognized Postgres contributors: the Contributors list [1], the
Recognized Contributors Policy page [2], and the Contributors
Committee governance page [3].

The Contributors Committee proposes doing this to make it more clear
that contributors may be nominated by contacting us and to make the
contributor criteria more discoverable from the contributors list
page.

I've included one change that is dependent on the request described in
Christoph's email [4] happening first.

- Melanie

[1] https://www.postgresql.org/community/contributors/
[2] https://www.postgresql.org/about/policies/contributors/
[3] https://www.postgresql.org/about/governance/contributors/
[4] https://www.postgresql.org/message-id/ZrYT4MPcoIjcVBTc%40msg.df7cb.de