Importing IntroToRx.com content

[LeeCampbell]

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Enhancement

This PR addresses #836 and #1563 .
Discussion has occurred offsite, and an agreement has been made for myself (@LeeCampbell) to hand over the IntroToRx content to the community as long as it retains attribution.

Further edits are underway, on the original IntroToRx site - LeeCampbell/IntroToRx#36
These edits can now be migrated to this site.

Note that the content that is being imported was in an early version of markdown that was used to generate HTML for IntroToRx.com and the Kindle book available on Amazon.
As such it has formatting and content that will no longer be relevant and can be omitted.

cc @HowardvanRooijen @idg10

[idg10]

Thank you very much for this @LeeCampbell

Documentation is hugely important, and Rx in particular has some complex concepts people need to understand, so it really needs good explanations of why it works how it does, and how to use it. So we're very grateful to be able to build on your work. Your IntroToRx site was a great gift to the community, and we are excited to be able to give it a new lease of life as the basis of the main Rx.NET documentation site.