Update and refresh cirq-* module README files

[mhucka]

This updates all the cirq-* module files to fix some broken image links, spiff up the layout and formatting, and add some additional links here and there.

As before, these remain purposefully short and fairly minimal because these are add-only modules to the main cirq distribution. The main package README will be much fuller.

Due to differences in reStructuredText support between GitHub and PyPI, some desirable attractive formatting cannot be achieved without using raw HTML, which PyPI does not support. I decided to focus on PyPI rather than GitHub for these README files; the rationale is that these modules are separate packages on PyPI and thus have their own landing pages there, which makes them more visible than on GitHub where they are buried in subdirectories of the overall Cirq repo. Consequently, these files look slightly better on PyPI than on GitHub.

Here are a couple of examples of what the result looks like on PyPI:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Coverage        ?   97.86%           
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  Files           ?     1084           
  Lines           ?    94299           
  Branches        ?        0           
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  Hits            ?    92281           
  Misses          ?     2018           
  Partials        ?        0           

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