standardize samples directory layout

[tswast]

We have an odd mix of sample locations:

• docs/snippets.py -- the original location, not very copy-pasteable / browsable
• lots of samples in samples/*.py, but the test drivers are in samples/tests/
• samples/snippets/*.py
• samples/geography/*.py

To align with the infrastructure in https://github.com/googleapis/synthtool/tree/master/synthtool/gcp/templates/python_samples, the goal is to have:

• samples/snippets/ -- these samples just require the google-cloud-bigquery library
• samples/geography/ -- require extra dependencies for working with geography data
• sample/pandas/ -- require extra dependencies for working with pandas dataframes
• more??? -- possibly a directory for the user-based authentication samples

To avoid breaking cloud.google.com docs, this needs to be done in several steps:

1. Copy samples to correct location
2. Update docs (Googler must do this)
3. Delete duplicate samples in old location

Towards: #366 (use templated noxfile) and #765 (blacken samples using templated noxfile(s))

[chalmerlowe]

This is a comment from another open ticket. Consolidating here and closing that other ticket.

Ideally we'd have some code samples which only require the bare minimum dependencies (no pandas, no google-cloud-bigquery-storage, no protobuf, etc). The samples/snippets directory would be perfect for this, but:

There are a couple of pandas-related samples there which ideally should be moved to a samples/pandas directory.
Likewise, there is a "desktop application" sample https://github.com/googleapis/python-bigquery/blob/main/samples/snippets/user_credentials.py which ideally belongs in its own directory with its own noxfile.py and requirements.txt. This is because it's responsible for the google-auth-oauthlib dependency, which isn't required for any other samples. Maybe we call this directory samples/desktopapp or something?
Related issues:

There are still code samples that haven't been moved to subdirectories: #790