Detect wrong spacing for RST directives

[javiereguiluz]

The idea is to report issues like this, which are minor but hard to see:

Source: https://github.com/symfony/symfony-docs/pull/16979/files

[OskarStark]

It's an easy one I guess. If you like you can copy one rule and add some tests. I will be back in September to work on it

[javiereguiluz]

I won't have time for this, sadly. If anyone wants to take it over, please do it. Thanks 🙏