Documentation/HOWTO: adjust external link references

- A few link references were missing http://
- Several sites are now redirecting to https protocol. On such
  cases, just use the https URL.

NOTE: all URLs were checked and they're pointing to the right places.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>