Add a Test Book to verify style changes against

[YJDoc2]

Currently, there is no standard way to checking, verifying and validating changes done to styles or alignments of elements. Most of the time, people notice bugs in some mdBook book, and they fix and verify the fix on that book, see #1617 . This adds a test_book in the repo itself, which contains dummy examples of various tags by themselves and mixed together ; as well as examples of language snippets to validate changes in highlight.js (see point 8 in https://github.com/rust-lang/mdBook/blob/master/CONTRIBUTING.md#updating-higlightjs)

This is a big PR, in terms of additions, but none of it is any code changes to source code. All of it is addition of a new book in a directory called test_book.

[YJDoc2]

This can be used as follows :
Run cargo run -- --serve ./test_book in root directory while making changes to styles etc, so that it will build it with the changes, and one can validate the style changes does not break anything.

[ehuss]

Thanks, this looks great and will be very useful!