docs: Type-safe code generated interface for future calls

[Crazelu]

Adds documentation for the type-safe code generated interface for future calls.

Closes: serverpod/serverpod#4607

[Crazelu]

@marcelomendoncasoares here's the PR for the future calls docs

[marcelomendoncasoares]

Thanks for the quick PR on the docs, @Crazelu! I have left a few guidelines about the experience we aim with docs on the comments, and also some improvement/fixes suggestions.

Beyond these, I missed a reference on the usage of abstract to declare future calls in modules, that users can extend in their own projects - since there is no way for users to mixture the ones generated in the module with their own. We should also present the support for inheritance of future calls in itself.

Another extra page that is worth adding is Recurring tasks to show how to achieve a cron-like scheduling. The current recommended way is by scheduling a future call inside another - at the start, for example. With generated future calls, though, it will probably be a two step operation or require the creation of two separate future calls: one for the unit execution, the other for invoking the first twice (now and next) to make it recurrent.

[Crazelu]

@marcelomendoncasoares the docs are updated now

[marcelomendoncasoares]

Excellent work on the docs, @Crazelu! Just two small details and we are good to go 🚀

[marcelomendoncasoares]

Great work, @Crazelu!!

Now we just need to merge main and copy the new docs to the versioned_docs/version-3.2.0 path, so it gets updated in the versioned docs.