Improve documentation around options and defaults

[josebolos]

The global options concurrency and useIncognitoBrowserContext are mentioned in passing in the Default configuration section of the README. For someone not familiar with pa11y this makes it look as if these options are part of the defaults that apply to each one of the tests, but they are instead global options exclusive to pa11y-ci.

We probably need to change the order of these options and add first a section with pa11y-ci's options, and then a second section or subsection dealing with specifying defaults for the tests.

Environment:

$ pa11y-ci -V
2.4.1