Problems with documentation TOC

[Disassem]

Several .. toctree:: commands in index.rst make following problems:

• On page security.html Previous topic is Contrib Modules instead of PoolManager
• Page Collections didn't include in "Next topic chain".

security included in main toctree twice (begin of index.rst) but exceptions not once.

Also there is one build warning: "urllib3\docs\security.rst:117: WARNING: duplicate label pyopenssl, other instance in urllib3\docs\contrib.rst".

[shazow]

@Disassem Any fixes/improvements to the docs are super-appreciated. Would you like to do a PR? :)

[Disassem]

Just now I don't know how to fix it. I'll think about this :)