Introduce a warning for forgotten Backticks

[Alagts]

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.

[garvinhicking]

We may also need to exclude escaped backticks like \` for some more false negatives. I like this idea!