Polish docs a bit

This is hopefully my last contribution before 14.0. :)

Notes:

- I’ve extracted a CONTRIBUTING.rst because that’s gonna be presented to everyone on github creating a pull request.  We should add more meat but it’s at least something.
- In the ldap intro, there’s a sentence saying: `An LDAP search takes the following information as input:` but then nothing follows.  Some copy pasta error from the original?
- I’ve somewhat reordered the TOCs, I find they make more sense like this.  I hope you agree. :)
- I have also fixed a few bugs/typos here and there.

Enjoy!