[DOCS] Add initial migration guidance for existing 7.x NEST users

[stevejgordon]

To make it less painful to migrate existing applications which use 7.x NEST, we should provide some general guidance on the availability of support for features.

Initially, it would be helpful to begin with the following:

### Migration documentation
- [ ] Create a "Migration" section in the documentation
- [ ] Include a sub-section with a table of all endpoints showing the version where support was added
- [ ] Include a sub-section with a table of all query types showing the version where support was added
- [ ] Include a sub-section with a table of all aggregation types showing the version where support was added

Later, we can focus on some worked examples of migrating code.