Graphic indicated crypt setup would be sent before client authenticated.

Protobuf project moved to github.

Update protobuf link

- Better description of the Opus/CELT packets (terminator/continuation
  bit).
- Fixed the general text from pre-Opus era.
- Better rst formatting for tables.
- Split some of the more confusing tables into multiple parts.
- Some section reordering for more logical order.
- Some other stuff I probably forgot.

Added configuration for using the RTD theme locally. THis requires the
rtd theme to be installed on the local machine.

See https://github.com/snide/sphinx_rtd_theme

Fixed the link to point to the current documentation and updated the
Mumble version number.

Voice Data & random tweaks

* Use it/its for the "client" to be consistent with the rest of the docs
* Use neutral pronouns (they/them) for user/speaker

Fix use of pronouns

Integrates our network protocol docs [1][2]. This will allow us to drop the separate repository. Git history is preserved through adding/merging with/through git subtree.

via

```
git subtree add --prefix docs/dev/network-protocol https://github.com/mumble-voip/mumble-protocol.git master
```

[1]: https://github.com/mumble-voip/mumble-protocol
[2]: https://mumble-protocol.readthedocs.io/

---

Add 'docs/dev/network-protocol/' from commit 'b0d142143c5c37797d787fd7f71eb172dd1219ad'

git-subtree-dir: docs/dev/network-protocol
git-subtree-mainline: 65462b4
git-subtree-split: b0d1421

Dropping the resources for Sphinx documentation generation drops support for generating this documentation into various formats (see make file; html, epub, and other less useful ones).

This network protocol documentation is the odd one out compared to our other documentation. The only one with this system.
Diverse tech has a maintenance cost, and the previous distance to our code made more likely to be missed for updates and discovery.
Handling only the network protocol documentation in this way feels disconnected/diverging.
Although readers may use the docs to implement clients and servers against only the docs, the question is whether the technology and maintenance additions are worth it.

Having simple structured text documentation files should serve us well enough.
It does for other developer documentation / documentation for developers.

In a folder, GitHub shows a README by default, below the file list.

Combining the index.rst and introduction.rst, with text fixups, will serve as an appropriate landing page/doc.