Improve protocols index page

Make the protocols index page more detailed:

- Explain what are services/protocols.
- Make it clear that we will only answer questions for protocols that come
built into Pidgin.
- Explain that for third party protocol plugins users need to ask questions
directly to their respective authors.

Testing Done:
Ran `dev-server.sh` and verified contents look as intended.

Bugs closed: NEST-33

Reviewed at https://reviews.imfreedom.org/r/820/