RST roles in titles render properly with rst_prolog

[picnixz]

When an RST file contains a section starting with an RST role and when rst_prolog is set, the prolog is inserted betweeen the title and the heading, causing the title to be incorrectly rendered.

For instance, if rst_epilog = ".. |foo| replace:: The foo", then

:mod:`foo` - |foo|
------------------
The foo module.

was actually transformed into

:mod:`foo` - |foo|

.. |foo| replace:: The foo

------------------
The foo module.

instead of

.. |foo| replace:: The foo

:mod:`foo` - |foo|
------------------
The foo module.

Fix #11437.

[AA-Turner]

@picnixz -- I've updated the methodology here to use a more correct regular expression, which means the previous logic within prepend_prolog() can be retained.

Note e.g. :field name: and :spam eggs ham:`code`:: are valid field names, but :role:`...` isn't. The bibliographic fields (docinfo) are all single words though, so we could restrict the regular expression if wanted, e.g. from: :(?![: ])([^:\\]|\\.|:(?!([ `]|$)))*(?<! ):( +|$) to :[A-Za-z]+:( +|$).

A

[picnixz]

Thank you ! I didn't actually think about using the docinfo pattern itself actually but your approach is much better.