Guides for projects related to OSBuild

Notes on the format:

• The guides use simple markdown files, which are rendered using mdBook: https://github.com/rust-lang/mdBook/.
• To render the guides locally run in one of the directories:

$ mdbook serve

Github Action:

• The "build-guides" workflow builds the book and stores the resulting html files in gh-pages branch in docs folder.
• Currently works only for the main branch.

Tip: If you prefer another markup language, use pandoc for translation. For example:

pandoc -f markdown -t asciidoc