User manual revamp

[Omikhleia]

Docs:

• Add backmatter = appendix with selected bibliography
• Migrate Markdown chapters to Djot = faster, more feature (more consistent choice also)
• Add an installation chapter = Requested by some friend. Reading GitHub README's is hard for non-tech people apparently. 😁 and such a chapter is expected in such a book. Well.
• Add an introductory chapter = Some general goals for casual readers that would go far into the manual...
• Review and expand some chapters = Also reusing (adapted versions) of images I had for a set of (unpublished) slides
• Reorganize most of the old class-related SIL content into Djot (part of Kill the SIL :) #73)
• Rewrite layout-related chapters in Djot (another part of Kill the SIL :) #73)
• Add an introduction to book/page layouts = with an illustration of all the "frames" that goes with a layout...

Fixes:

• Revert an erroneously changed line in styles = marked as "chore", the issue is only on the main branch not in a released version
• Some more better support for null in YAML style files.¹
• On the way, small bugs were found in ptable.sile and markdown.sile, so there's a dependency update...

Features:

• Add several predefined TOC-like symbols for Djot docs = A minor thing on the way, getting rid of direct SIL commands in the manual too.

Closes #73

Footnotes

1. Bugs occur if a null is used on terminal leaf fields. This may happen when using some advance styling, and I started doing that in the docs: The null allows for explicit cancellation of inheritance on these leaf fields, which is kind of handy. I don't like the implementation, though, it's some monkey-patching for now. Styles will require a clean-up / refactor at some point. It's a compromise, trying to make the code work while toying with it and pushing it to its limits in the manual... ↩