Add documentation

[cbm755]

We've talked in various places about adding more documentation, like a manual or FAQ. Here's tracker bug so we don't lose those things.

See #104.

[catch22]

I think that would be useful. Some things that come to my mind:

• The various syntaxes and in which situation they are appropriate (More explicitly document the two kinds of syntaxes #104).
• How to specify the various doctest targets (e.g., doctest @class vs doctest class doctest class versus doctest @class #88).
• Other best practices ("How do I specify tests that rely on Matlab/on a third-party library/...?").

[cbm755]

• Combinations of XFAIL_IF comments: Should XFAIL_IF need parentheses? #141 (comment)

[cbm755]

• XFAIL_IF: Downside is the conditional code cannot contain the chars # or %: this is not anticipated to cause much hardship.

[catch22]

• Related: Terminology bike-shedding in Rename doctest_collect #128