Add model schema in guides about symbol-docs

[dgarcia360]

Thanks @daoka for your contribution!

Currently, we are missing most of the model schemas that you provided in the current documentation.

Instead of linking it as a guide, I think it would be more consistent if we copy each schema to the "built-in feature / protocol" section where it belongs.

What do you think ? If you want, you can apply directly the changes by submitting a pull-request. On the other hand, if you don't have the time I can create a new branch, organize the model schemas in each document and add you as a co-author of each commit improvement.

[daoka]

Instead of linking it as a guide, I think it would be more consistent if we copy each schema to the "built-in feature / protocol" section where it belongs.

It's a good idea.
I'll add each schema to the "built-in feature / protocol" and submit a pull request.