Book evolution in main Bevy repository

rfcs/70-book_evolution_in_main.md

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+# Feature Name: `book_evolution_in_main`
+
+## Summary
+
+Reduce the cost of starting the book, and of ongoing book evolution with new versions of Bevy.
+
+## Motivation
+
+I want the Bevy Book to get better faster. We've tried to make it evolve in bevy-website repository, but getting to a publishable state means getting the content, the CI and the formatting ready all at once and lead to a long lived `new-book` branch that is in limbo. The current approach would also mean that for a new version of Bevy, we would have to pull a new branch, make it use the next version of Bevy, update everything, then merge it back on the website for publication. I fear this will make the release process longer.
+
+I'm opening this as a RFC as it has moving parts across 2 repositories, and it's an evolution on [RFC #23: Quick Start Book](https://github.com/bevyengine/rfcs/blob/main/rfcs/23-quick_start_book.md).
+
+## User-facing explanation
+
+As a user wanting to contribute to the book, I can do so on the Bevy main repository.
+
+## Implementation strategy
+
+- Add a new folder in Bevy main repository for the book
+- Start accepting contributions for it, with the layout / rules from [RFC #23: Quick Start Book](https://github.com/bevyengine/rfcs/blob/main/rfcs/23-quick_start_book.md)
+- Set up main repository CI to check that the book content is up to date with the code
+  - For new book PRs, this should be blocking
+  - For new code PRs, this should be a non blocking CI. Writing or updating the book can be delegated to another issue that will be added to the next milestone
+- Set up bevy-website repository CI to pull the book content from the latest tag and publish it
+
+## Drawbacks
+
+🤷
+
+More non code commits in the main repo maybe? But I think that's good as it gives better visibility to book contributions. The main repo will also gets larger which can slow down checkout / clones.
+
+## Rationale and alternatives
+
+- It allows us to start writing the book without worrying it about being public directly, or working in a long lived branch
+- This keeps the book versioning in check with Bevy
+
+## Future possibilities
+
+- Publish the book content as part of the rust doc (a `bevy_book` crate?)
+- Publish the book content with versioning
+- Remove `/docs/` in the main repo and migrate all of that content to the Bevy book