Improve the structure of the mdBook documentation

[David-Else]

(This is a follow up to #5534)

The current documentation does not have a consistent structure making it hard to see an overview of the content and navigate to the desired section. Currently:

1. Installation is totally self-contained with no mdBook subsections. All navigation is in the TOC.

2. Usage has some usage guides in the markdown file TOC, and also some in mdBook subsections.

3. Migrating from Vim is not complete and basically just a link to the WIKI < deal with that another day

4. Config has the Editor subsection in the markdown file (no TOC), and then the rest in mdBook subsections.

5. Guides is the only consistent one. A brief description of the section, and then all neatly divided into mdBook subsections.

I have refactored it to be consistent and follow the following format, the same as the current Guides section:

x Section Name

  Contains overview of the section, or important info that does not merit a subsection

    x.x Subsection Name

      Contains the content

I have not changed any of the actual content, apart from in keymap, the heading should be an h2 for consistency, and - [Comment mode](#comment-mode) was a bug/broken link, the content never was created.

[David-Else]

@the-mikedavis Can I request a review from you? I can't seem to find how to do it in the Github web interface, cheers.

Here is how it looks now: