Create extensible points concepts doc

[dotsdl]

We should create an "Extensible Points" concept doc that at the very least lists the extensible objects featured in gufe:

• Components
• Protocols
• AtomMappers
• others?

For these, we want to lay out the context of these objects:

• what information do they contain?
• where are they typically used?
• how are they related to other components in gufe?

For each object, we should also link to relevant how-tos, as well as the relevant reference pages.