Introduce a warning for forgotten Backticks

Feature request

To increase the possibility to find forgotten backticks (during local rendering or before someone is going to push new changes) like in here :file:`.rst.txt' we can build in a check that examines if the number of all backticks in a rst file is even. It is clear that two forgotten backticks make the number even again. Maybe we can catch it somehow. And we have to do it in the rst parser in order to exclude backticks in code blocks , comments and everywhere else where it could spoil the calculated number of backticks.

Start by locating the RST parser and trace how it distinguishes normal text from code blocks and comments. Add the requested backtick check without counting excluded contexts, and verify that forgotten backticks produce a warning during local rendering or before changes are pushed.