Improve mupdf-sys docs

[ginnyTheCat]

Parses the mupdf docs and converts them into a more docs.rs friendly format. It links other types and functions and puts some parameter names in markdown inline code apostrophes. It also escapes <, >, [ and ] as docs.rs will interpret them as HTML or links otherwise, causing the <path> tag to disappear as in the pink highlighted example below.

It also splits at every linebreak, which sometimes looks a bit unnecessary, but cases like the last example from the table below are otherwise unreadable.

Because of the very different styles of comments across the mupdf codebase some weird formatting issues might happen with this change, but I've looked for these cases and haven't found any yet.

Here are some comparisons:

Before	After