docs: work minisite

Adds minisite at docs/html/work. Adds landing page and
Android for Work specific guides. Moves docs from
training/enterprise and add redirects. Update resource
card info and add new images. Update sitemap, _book, and
add sidebar navigation menu.

bug: 27744376
Change-Id: Ib09f7fc6352dfde7a60fc674b06488c651b196e2