docs: misc issues and improvements

[bshaffer]

More improvements are required on the docs to better support the new surface

1. The published docs have duplicate entries for magic methods. For example: https://cloud.google.com/php/docs/reference/cloud-automl/latest/V1.Client.AutoMlClient

2. new and previous clients aren't ordered predictably (e.g. they should be new and then previous, or vise versa, consistently - see AutoML TOC)

3. we still don't include the namespaces, and this is even more important with the new clients (tracked in feat: add support for namespaces in the HTML template doc-pipeline#478)

4. there are no samples for the new clients - we should include these with the @example annotation when we can!

5. the new clients, which are in beta, are not labeled as such

6. the __call method shows up in the docs, and we don't want it to

7. We should include information in the top of GAPIC clients regarding the migration status to help users on libraries which contain both surfaces

Here's an idea for how we could ptentially support namespaces

[bshaffer]

• duplicate entries (done in fix: dupe magic methods for async methods #6322)
• new vs previous client ordering (done in chore(dev): sort beta clients to bottom, remove new and previous labels #6330)
• namespaces added to templates (done in feat(docs): add namespace (formatted for docfx) #6332)
• samples for new clients
  • Add support to GAPIC generator (done in feat: add phpdoc examples tag gapic-generator-php#636)
  • Add support to DocFX generator (done in feat(docs): support examples phpdoc tag for methods #6328)
• mark new clients as beta in TOC (done in fix(docs): mark classes with experimental tag as beta #6329)
• remove __call method from docs (done in chore(docs): exclude call method #6337)
• more information for migration status (done in chore: add migration information to service class descriptions gapic-generator-php#638)

[bshaffer]

Confirmed these have all been fixed and published.