[feature] Add grammar checking to openwisp-qa-check

[nemesifier]

It would be great to add the capability of performing grammar checks on restructured text or markdown files (eg: README.rst, README.md or any text file in docs/ directory).

A quick look found me https://caderek.github.io/gramma/, it seems it may need an external server or maybe a local server can be spawned.

It may not be so easy to add this but if we can do it in a way that doesn't slow down the check process too much it would surely be beneficial for the whole project.

[nemesifier]

Moving this to the docs repo, we can add the spell checker there.