Add schema building docs (basic, hierarchical, rule)

[kasper-piskorski]

No description provided.

[sorsaffari]

@kasper-piskorski Following the discussion we had about this PR, I'm going to close it.

The conclusion of our discussion was:

• providing schema examples that include type hierarchies and rules should be a part of the content for the reasoning section.
• the content on use-cases of rules in a schema should be referenced from the graql/schema/rules section.
• the content on use-cases of type hierarchies should be referenced from:
  • graql/schema/overview, and
  • graql/schema/concepts.

Before you start writing the content on Reasoning, let's have a discussion to outline the main points we want this section to deliver.

[kasper-piskorski]

reopening to make sure i dont forget about the content

[sorsaffari]

@kasper-piskorski

Can you create an issue to help you remember about the content, and close this PR?