Added 'CUSTOM_DOCS' support when using 'CUSTOM_DOMAIN'

[tylerdotrar]

Original Issue:

• When using CUSTOM_DOMAIN with an MkDocs project with a custom docs_dir (e.g., my RGBwiki project), deployment fails because actions.sh was statically set to add the CNAME to the /docs directory; meanwhile the mkdocs.yml file is expecting a different directory.

Fix:

• Added an environment variable called CUSTOM_DOCS that allows you to to specify the docs_dir to echo the CNAME to.
• If CUSTOM_DOCS is not used, docs is the assumed path.

(Note: also bulleted and added to the README.md to make it easier on the eyes)