docs: Allow multiple automarkup functions

The automarkup script previously matched expressions and substituted
them with markup to enable automatic cross-reference all in the same
function.

Split the expression matching iteration and the markup substitution into
different functions to make it easier to add new regular expressions and
functions to treat each of them.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20200911133339.327721-2-nfraprado@protonmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>