📖 docs: Add book for CAPRKE2

[yiannistri]

What this PR does / why we need it:

This PR adds a book for CAPRKE2. As part of this PR, I have moved the getting started and the developer guide from README.md into the book and provided links to the relevant pages instead. I have also added support for using the embed-github syntax for convenience although it has not yet been used. Finally, I have added a GHA workflow that deploys the book to GitHub Pages.

Which issue(s) this PR fixes:
Fixes #92

Special notes for your reviewer:
You can run cd ./docs/book && make serve from the root of the project to be able to view the book using your browser.

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[furkatgofurov7]

Thanks for putting this together @yiannistri, this serves as a foundation for the provider book in the future.

I know we lack documentation in this repo and should do better, however, may I ask you to have a simple structure for the book (or even similar to https://cluster-api-operator.sigs.k8s.io/ for inspiration) so in the future we can document more stuff in the corresponding folders of the book. We can start simple, having a basic skeleton that is kind of similar to the upstream ones but extendable, is all I am after here.

PS. mdbook setup seems to be working and serving the book properly when trying it locally.

[furkatgofurov7]

Great job @yiannistri, this version as a base of the provider book,LGTM!

[alexander-demicev]

thanks!