Fix main docs location

[pquentin]

Using the ReadTheDocs badge isn't appropriate, but I wanted to share that finding the new DSL docs isn't easy today:

• Searching "Elasticsearch DSL" pointed me to https://elasticsearch-dsl.readthedocs.io/en/latest/ (other searches get me to the elastic.co docs, but that's not the one I did)
• This links to https://github.com/elastic/elasticsearch-py first (I was too lazy to read everything and notice the correct links)
• This contains "docs" badge pointing to https://elasticsearch-py.readthedocs.io/en/v9.1.3/
• The DSL docs at https://elasticsearch-py.readthedocs.io/en/v9.1.3/dsl.html don't link to the narrative docs.

Suggestions:

• GitHub doc link should not link to API docs
• DSL API docs should link to elastic.co docs
• DSL API should be named "Elasticsearch DSL API", as was done to "Elasticsearch API". This will clarify the scope at the Google search level.

[miguelgrinberg]

Thanks!