Website: restructure the Foundation substructure

[june-andronick]

(This may wait for the Information Architecture feedback first)

• The Foundation substructure should only keep these pages: About, Governance (see point below), Governing board, TSC, Membership and Join.

• I would merge the Governance page into the About one; I would remove the "key principles" document (outdated), just keep the Principles paragraph and have a "Two Foundation bodies" paragraph pointing to the Governing Board and the TSC (adding the currently missing link to the TSC page). I would update the About page, removing old info like the original press release.

• The following other pages aren't specific to the Foundation and already appear on the main landing page: Summit, TM, Support seL4 / Funding the next big steps..., Commercial support / Endorses providers. I would therefore just remove them from under the Foundation, but we need to revisit the top menu bar do have them appear somewhere (Website: top menu should be reordered and restructured #305).

• The following 2 pages only appear under the Foundation: Jobs and Contact the Foundation. The Contact page should be removed and link to the general "Stay in touch" page. The Jobs page is out of date and should be revisited (Website: jobs page is outdated #306)