phase out wordpress archive page in favor of archive.s.o

The archive page on wordpress and the webapp homepage are highly redundant. We want to merge the former into the latter.

In concrete terms:

the Archive label on the wordpress navigation bar should be made to point to archive.s.o
the growth graph should be integrated into the archive.s.o homepage
a call to action pointing to https://www.softwareheritage.org/support/ should be added from the archive.s.o homepage
www.s.o/support should be revamped to be less bare-bone
www.s.o/archive should be removed

Migrated from T2008 (view on Phabricator)

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