Better Content Block fieldtype support

[kylecotter]

Content Block fields have come in super handy lately. The Craft skill has trouble interacting with these fields when it comes to creating and updating these types of fields.

Part of it could be related to the Matrix Field example referring to them as "Flexible Content Blocks". It would also benefit from having specific examples of what makes a content block fieldtype unique to other fields in that it has its own field layout designer that is not an entry type.

[markhuot]

I've been building out a similar set of LLM guidelines over on the Craft Pest repo, https://github.com/markhuot/craft-pest-core/tree/main/llms/field-types. I think the same is necessary here. A lot of the existing .md files in this skill are about managing schema but there's virtually no guidelines about entering content.

We can definitely make improvements here. My initial thought is that create_entry.md and update_entry need to link to a custom_fields.md file that describes how custom fields work within the Craft SDK. And then from there link off to custom_fields/matrix.md and custom_fields/content_block.md etc…