Docs: Document block validation, clarify extensibility validation

[aduth]

Closes #4759

This pull request seeks to add documentation for block validation, describing the process for determining whether a block is invalid, options presented to the user for resolving the invalidation, and debugging steps for plugin authors. Further, it clarifies a confusion about block extensibility and its relation to validation.

Testing instructions:

There are no behavior impact expected with the changes proposed here.

[youknowriad]

Validation is also tied to the deprecated blocks API which is documented on a separate page right now. I wonder how we could better organize all these related docs.

[aduth]

Validation is also tied to the deprecated blocks API which is documented on a separate page right now. I wonder how we could better organize all these related docs.

Maybe it'd be enough to add a question to the FAQ section here about "intended invalidations" ?

I changed my block's save behavior and now old content includes invalid blocks. How can I fix this?

Refer to the guide on Deprecated Blocks to learn more about how to accommodate legacy content in intentional markup changes.

[youknowriad]

Maybe it'd be enough to add a question to the FAQ section here about "intended invalidations" ?

@aduth I like this proposal 👍

[aduth]

Updated in 94902d9