docs: fix all README samples, rename command

[bshaffer]

• Ensures most recent version is used (e.g. a V2 sample will be used before a V1 sample)
• When ### Sample doesn't exist in the README, inserts the sample above ### Debugging instead. This will add samples for many READMEs which previously were being skipped because they didn't contain a ### Sample header
• Fixes the bug where all README samples were missing use from import statements.
• Updates ALL README SAMPLES with these fixes

[bshaffer]

We are going to save this for next week when we roll out version 1.20 of the gapic generator (see googleapis/gapic-generator-php#768), because that will also update the docs for every single client library.

[Hectorhammett]

This looked good to me but we need to re review after we see all the updates from the generator. Waiting on that update.