Documentation for inline layout algorithm

[d-desiatkin]

Motivation

Existing online crate documentation mostly follows CSS specification: CSS Inline Layout Module Level 3, part 2, Inline Layout Model. But this is very high level overview. I belie that a number of important details should be explained for newcomers to web technologies domain.
Lets add documentation on such things as:

• Box tree and Inline Formatting Context construction
• Bidi processing and reordering
• Text shaping
• Line breaking Documentation on Inline items linebreaking #76
• Fragments generation Documentation on fragment generation #77