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More proxy documentation.

* Add a table of contents
* Use "proxy server identifier" throughout
* Rework the proxy server identifier section
* Use URI formatted proxy server identifiers throughout document
* Add some links between sections

Change-Id: Ie61212697cf70a2532d971113dbd550639ae9f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636259
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665608}