Recommend pip to install plugins

[progval]

https://docs.limnoria.net/develop/plugin_distribution.html#via-pip-pypi has been out for almost two years, it's time to:

1. consider it stable
2. migrate third-party plugins' install instructions to recommend it
3. rewrite PluginDownloader to use pip instead of raw git clones when possible
4. eventually deprecate PluginDownloader

[koobs]

Suggest suggesting/recommending:

• Running (limnoria) in virtual environments (vs system), showing also:
• How to inherit system site packages (to benefit from OS packages)
• Running pip as user (not root), if not using virtualenvs (to avoid polluting system site-packages

Basically a simple 'easy and best practice deployment' guide.

[progval]

Running pip as user (not root), if not using virtualenvs (to avoid polluting system site-packages

Python took care of that one with PEP 668