V2: Extend documentation for descriptors

[timostamm]

In the guide for writing plugins, we explain the descriptor hierarchy, and how to walk through it.

It's great, but now that descriptors are more prominent in v2, the hierarchy should be explained in the section about descriptors instead.

This PR adds

• a description for every descriptor type
• the section "Walking through a schema"
• the section "Walking through message fields"
• the section "Field descriptors"

As a follow-up, we can update the guide for writing plugins to condense it a little bit, and link to this new documentation where applicable.