3.0 API Migration guide

From original PYCBC-682:

A complete guide to all the changes in the API will not fit in a single document.
Instead, the migration to the new SDK doc should concentrate on a couple of key areas -
giving before and after examples of operations, then tips on key changes to watch out for.

Please work directly with Richard Smedley to ensure the doc for this SDK best addresses issues that customers are likely to come across during migration.

To go here: https://github.com/couchbase/docs-sdk-python/blob/release/3.0/modules/project-docs/pages/migrating-sdk-code-to-3.n.adoc

CONTENT:
After discussion of key changes in object model caused by move from Cluster to Scope/Collections,
and the fact that we've moved from returning a document, to returning a response....

Code samples of before and after for: KV (CRUD), Query, and Analytics, including error handling.
plus "anything else that is important in this SDK".

From later PYCBC-689 description:

Please think of what the Python developer needs to know most, and prioritise this.

• Common SDK content: https://github.com/couchbase/docs-sdk-common/blob/release/6.5/modules/shared/partials/migration.adoc
• what it looks like together: https://docs.google.com/document/d/1S0oYzEsAEJnS2n1sgxSrzgg8h6CQPDCr6Q69cg1Ma3w/edit#