doc: add support for linkcheck

This patch adds support for the `linkcheck` Sphinx builder, so that we
can easily check for broken links in the documentation.

It can be run like this:

ninja linkcheck

Or, using the Makefile shim:

make linkcheck

All Zephyr Github issues links are ignored, since we have lots of these
URLs and they quickly hit GH rate limit. They have small chance to be
incorrect, so we should be ok.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
3 files changed