docs: do it with style: interact with the bot

[Exirel]

Description

As many questions have arisen on IRC about formatting and stuff, I figured I could improve the documentation about that. There is now a new section in the chapter "Interact with the bot" about working with the bot "with style".

Reviewers may be confused as to why I had to add custom roles and CSS class, but worry not, that's the only way to do it properly.

We may want to add further warnings about control code, because formatting in IRC depends on the client's implementation.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

Because I know this will get posted on IRC: @Exirel I found the energy to review #2013 now. 😛

[Exirel]

All fixed!

[dgw]

Yep, this is just about ready. Feel free to squash at the same time as you apply these tiny little suggestions (put [skip CI] in the commit message when applying, so Travis won't try to build until you force-push the squashed version).

[Exirel]

Done.