doc: use absolute links in style guide, add link to cookiecutter

[petrasovaa]

Relative links to files in repo don't work anymore now when style guide is part of documentation.

Also adds a link to the addon template in development documentation.

[echoix]

To see if setting docs_dir to a relative path like "../../" (appropriate for the location of the root of the repo with respect to the config file location).

https://www.mkdocs.org/user-guide/configuration/#docs_dir

Also consider if there's blockers to have the mkdocs.yml config file at the root, to be able to build with "mkdocs build" without changing any directories first.