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[Media Router] Media Router design document

This converts the design document hosted on sites [1] to Markdown and
makes a number of smaller fixes, including:

- Line wrapping and fixing Markdown formatting
- Updating links and contact information
- Adding a document history
- Removing sections that are no longer relevant and a few smaller edits
  to improve accuracy

Some sections need more thorough editing or need to be rewritten
entirely, and placeholder material inserted for other internal design
docs (1-UA mode, Mirroring Service, etc.).  Also several screenshots
and diagrams need to be updated.  All of these sections are marked with
TODOs and will be addressed in future patches.

Once this is landed the current document will be marked deprecated and
a link added to this document.

[1] https://www.chromium.org/developers/design-documents/media-router

Change-Id: I328fcd0c2fdc2f3c00a909cc700468ee09d2a240
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1673858Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676099}