Document `Topology.from_pdb` in standalone showcase/cookbook/example

[mattwthompson]

Is your feature request related to a problem? Please describe.

We get numerous, numerous questions about how PDB files can be loaded, what the behavior of the infrastructure is, what happens if particular things are found in the file, what the structure of the resulting Topology is, etc.

Describe the solution you'd like

I want to link users to an instructive, standalone example that guides them through the questions that they predictably have.

Describe alternatives you've considered

• Typing out the repeatedly the answers to the same questions (unacceptably poor scaling)
• Linking to docstrings (not appropriate for many users)
• Linking to release notes (insufficient information)

[mattwthompson]

I'll start this by splicing out the second half of this notebook, starting at "Loading PDB files," which was originally authored by @j-wags