howto-compile-linux: Use cgi-bin/latest to download

The cgi-bin/latest will return the latest tarballs, .sig and SHA512SUMS
files.  Use curl(1) for a shorter expression; wget(1) (v1.13.4) does not
have a short-option for --content-disposition.

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
website: Update download URLs for cgi-bin/latest

Other:
* Drop a HOWTOs anchor

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
website: shared/repo.txt: Update github note

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>