Remove "documentation" page

Most of the docs are under the "docs" directory. Whilst the "docs" still
need to be sorted into the new categories, by moving the remaining
document not in this directory into it and patching up links, we can
remove the documentation page. Hugo automatically creates an index page
for us, so this simplifies the website a little with no real loss.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>