Add documentation for ProxySettingsPolicy

[ciarams87]

As an NGF user
I want a guide on how to use the ProxySettingsPolicy
So that I can configure nginx proxy settings properly.

Acceptance:

• Create a guide with examples for ProxySettingsPolicy

Dev notes:

• example doc
• consider if we should reorganize the traffic management section of the docs (can be a separate task)

[ciarams87]

nginx/documentation#1639