Use on_files instead of prebuild

[athackst]

This feature updates the files in on_files to point to the original .md file if it exists and copies the file into a temporary build directory.

By updating on_files to point to the original file, plugins such as git-revision-date-localized work as expected.

For other plugins like awesome-pages, the files must all exist within the "docs" directory structure, so the plugin continues to copy these files to the build directory.

Major changes:

• The docs_dir is no longer updated to be the build_docs_dir. You may need to update your config so that the build_docs_dir and the docs_dir are the same for plugins that require file structure.
• The build_docs_dir directory is no longer cleared at each "config"/reload. If a user has set the build directory, they are now responsible for clearing the folder as needed. If the user has not set the build directory, a new random folder will be created in /tmp on each invocation. This may have some minor consequences for the local serving of the mkdocs site.
• Build now happens in on_files instead of prebuild which may affect the order of plugins.

[athackst]

@gwhitney Not sure if this will cause issues for you

[gwhitney]

Thanks for the heads up! I am sure I will need to make changes but I am sure I can adapt. So I will wait til you release and then I will update my mod. :)