Improve the "Getting Started" section with content from Knowledge Base

[davidhorstmann-arm]

Improve the "Getting Started" section of the docs. Specifically:

• Split "Downloading" and "Building" into separate pages
• Expand the "Building" page by folding in the content of a page from Knowledge Base

Note to reviewers: I think this is easiest to review commit-by-commit, as the transformations seem to make more sense that way.