Rename Contributor to Significant Contributor

Hi,

I plan to commit the attached patch to rename Contributor to
Significant Contributor. Also included is some copy-editing of the
pages.

https://www.postgresql.org/about/policies/contributors/:

* really needs a "Policy" in the page title or everyone is confused
why this page isn't listing people

https://www.postgresql.org/community/contributors/

* I added a "PostgreSQL" to the (in-page) title so "PostgreSQL
Contributor Profiles" sounds better than a mundane "Contributor
Profiles" listing

* I moved the "how to suggest" section to the end so the list isn't
cluttered with too much text at the top

* The sections all have introductory text now (likely needs manual
importing of the json file, like the actual "Significant" (and Past
Significant Contributor) rename.

https://www.postgresql.org/about/governance/contributors/:

* add Floor (already pushed in a separate commit)

Christoph