Add version into generated files

[felixfontein]

The generated HTML by Sphinx contains something like <meta content="2.3.1.post0" name="antsibull-docs">.

[samccann]

Because a year from now, I'll never remember the reason for this change:
the idea is that you can easily check when looking at HTML which version of antsibull-docs was used - this can be helpful when figuring out whether some dependency update really worked, or whether some broken layout might be related to a certain antsibull-docs version

[felixfontein]

This was discussed at today's DaWGs meeting, we agreed that the version should always be inserted for now. If someone comes up with a good reason why it should be possible to disable it, we can weight that reason against the additional complexity a switch needs.