Docs: Update base system requirements for Evergreen

Docs: Update base system requirements for Evergreen

These memory values are woefully out of date for modern Evergreen installations.

Recommending also at least 2 GB of RAM for client workstation, though honestly,
more is probably better there too.

Also adding specific Linux distributions supported by this community instead of
just Linux.

And dropping specific Windows versions (especially since two out of the three
are no longer supported by Microsoft).

Signed-off-by: Ben Shum <ben@evergreener.net>