Update docs index to include missing files

[JakeGinnivan]

There are a few pages in the docs which are not included in the mkdocs index file. Like https://github.com/GitTools/GitVersion/blob/master/docs/versioning-mode.md

We need to ensure nothing is missing

[asbjornu]

#989 adds versioning-mode.md to mkdocs.yml, but we should have a test that ensures all .md files in the docs directory are included, yeah.

[JakeGinnivan]

Nice.. Missed that one