Update Swagger UI section in README

[cremor]

Update a few outdated links to the Swagger UI repository and add missing extension methods on SwaggerUIOptions.

I also thought about adding mentions to the properties directly on SwaggerUIOptions (e.g. HeadContent) but decided against it for now. Those would need some additinal warnings because otherwise you might undo your previous modifications done by the extension methods again.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (a29ba36) to head (a0ea07f).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2883   +/-   ##
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  Coverage   90.06%   90.06%           
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  Files          72       72           
  Lines        2698     2698           
  Branches      426      426           
=======================================
  Hits         2430     2430           
  Misses        268      268           

Flag	Coverage Δ
Linux	90.06% <ø> (ø)
Windows	90.06% <ø> (ø)
macOS	90.06% <ø> (ø)

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[martincostello]

Thanks. I wouldn't do any more PRs to add further new content (tweaks/fixes are fine) - we think the README has gotten too big at this point and want to rework it. We just haven't worked out exactly what we want to do instead yet.