Add support for Gitlab Pages and move some data from README

Problem/Motivation

The default gitlab_template now has support for GItLab pages.

Working on #3403263: Require plugins to be responsible for locking it also appears that CR's will likely not be sufficient to ensure developers are aware of their responsibilities. Adding to the README itself would add more information that is not relevant to non-developers.

The D7 release has some well-formed docs on D.O. https://www.drupal.org/docs/7/modules/two-factor-authentication-tfa however no such documentation exists for D8 releases.

Using mkdocs we can keep the documentation with the source code and update it as code changes with the latest release version being published to the web (or in a more complex workflow, all versions published). As the docs are primarily Markdown they would still render in GitLab and would be readable text for site owners in their local install directory.

Security Concerns:
Documentation could be used as another technique to fingerprint the installed version of TFA. We may want to consider if we wish to remove the docs folder from published releases or if we leave that to site administrators such as with https://www.drupal.org/docs/develop/using-composer/using-drupals-vendor-... or #3401562: Add script to reduce disk usage by removing unnecessary files. This could be included in the new documentation as well.

Steps to reproduce

N/A

Proposed resolution

Setup basic framework for mkdocs, choose theme, migrate extraneous docs out of README and into specific locations.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None