Raise error when mkdocs-monorepo-plugin is loaded before this plugin

[timvink]

This plugin uses the config["docs_dir"] to find the .git folder. The monorepo plugin changes the docs_dir to a temporary folder. More context here backstage/mkdocs-monorepo-plugin#13 (comment)

We should raise an instructional error to change the plugin order in mkdocs.yml, so this plugin is always loaded first.

Examples:

Good:

plugins:
  - git-revision-date-localized
  - monorepo

Bad:

plugins:
  - monorepo
  - git-revision-date-localized

[timvink]

#41 will solve this, as repo will be determined per not using config["docs_dir"] but page.file.abs_src_path instead.