feat: Add resilient master file document basic support (1.0)

[Omikhleia]

Closes #18

To do:

• [x] master document file inputter 1.0 (minimal working)
• [x] master document file syntax check (schema)
• [x] documentation
• [x] rockspec update
• [x] check what happens when one \includes a master document in another document (lol): fix and document
• [x] working example - or try using it for the resilient documentation, and get rid of SIL as much as doable
• [x] Improvements on initial master document prototype
  • [x] check how to handle book parts (see comment)
  • [x] filenames in content section might not be sufficient, we may need an object (file, format) in case the extension is not one expected and the appropriation logic fails

Moved out of scope:

• possible refactors (utils, etc.): moved under the umbrella of #50 as it encompasses more things than addressed in this PR

[Omikhleia]

Comment on parts: Jupyter has this interesting structure -- https://jupyterbook.org/en/stable/structure/toc.html#use-parts-to-organize-chapters

format: jb-book
root: index
parts:
  - caption: Name of Part 1
    chapters:
    - file: path/to/part1/chapter1
    - file: path/to/part1/chapter2
      sections:
      - file: path/to/part1/chapter2/section1
  - caption: Name of Part 2
    chapters:
    - file: path/to/part2/chapter1
    - file: path/to/part2/chapter2
      sections:
      - file: path/to/part2/chapter2/section1

And automatic header shifting for Markdown -- https://jupyterbook.org/en/stable/structure/sections-headers.html#how-headers-and-sections-map-onto-to-book-structure -- although they use a “next-header-deeper” nesting strategy which I find weird.

There are some good ideas to consider there.