doc: add links to 1.10 docs and release notes

Add link to the 1.10 technical docs to the available tech docs listed in
the documentation home page. Add the 1.10 release notes to the list of
available release notes in the release notes page.

Also update description of where to get Zephyr source code (downloadable
archives listed in the GitHub tagged releases page).

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>