Consider moving IntroToRx into its own repository

First of all, thank you so much for the huge effort put into the development and updating of the Intro To Rx book. It is a massive resource for the Rx community.

I am raising this issue mostly for organizational and maintenance purposes. I feel the continuous development of the IntroToRx book should be kept separate from this repository for a few reasons:
- **Rx-focus:** this repository includes Ix in addition to Rx so there is asymmetry
- **update guarantees:** while I understand the motivation to have the book track the code and vice-versa, in practice I don't think this is achievable. A genuinely useful book simply requires digestion at a completely different time-scale than the code base, by both readers and writers. By definition it will require updates which are asynchronous to package release cycles.
- **search**: because the book is mixed in with code, it severely impairs GitHub search potential. If I want to search for e.g. `SubscribeSafe` inside the book contents, this is made unnecessarily hard since I will get a list of all the uses of this method in the code base which easily shadows any possible in-book references.

There may be more pros/cons which I may have missed, but these three I believe are already enough to warrant a more dedicated strategic discussion about this.

No files, tests, or entry points are named; start by reviewing how the IntroToRx book is currently organized within this repository and the three organizational concerns raised in the issue. Done would require a maintainer-approved decision on repository separation, with the resulting scope and migration plan documented.

Written by the indexing model from the issue text.