docs: fix usage guide

[daniel-sanche]

The usage guide in the docs page had incorrect information, and it contained samples that weren't part of our samples tests. I moved the samples in with the others, and updated them to all work properly.

Long term we should aim to keep most of this usage information in the official gcp docs and link there (or at least use the same samples), but this guide seems like it's still useful for now

Fixes #135 🦕

[snippet-bot]

Here is the summary of changes.

You deleted 6 region tags.

• docs/usage.rst:65, tag client_list_entries_multi_project
• docs/usage.rst:90, tag client_list_entries_paged
• docs/snippets.py:50, tag client_create_default
• docs/snippets.py:57, tag client_create_explicit
• docs/snippets.py:89, tag client_list_entries_paged
• docs/snippets.py:109, tag client_list_entries_multi_project

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