Extend the Sphinx docs by a page about the concepts of ConfigUpdater

[FlorianWilhelm]

Describe your use-case

Right now many features of ConfigUpdater are only explained by some examples in Spinx Usage page and thus it's cumbersome for new users to really understand how to use ConfigUpdater.

Describe the solution you would like to see for your use-case

Add a Sphinx page that explains the concepts of ConfigUpdater and which methods should be used in which context. The concept is basically that there are Container objects holding Block objects. ConfigUpdater itself is a Container and also Section. Option, Space, Comment but also Section are Block objects.

Now there are several methods for:

1. navigation, e.g. first/last_block of a container or next/previous_block of a block
2. creation, e.g. add_before/after.comment/option/space
3. removal, e.g. block.remove
4. inspection, e.g. option_blocks
5. etc.