Update documentation for v0.7.0 release

[jmduarte]

Description

Updating documentation for v0.7.0 release.

Checklist

• [x] Set up skeleton @jmduarte
• [x] Update release notes @jmduarte
• [x] Update contributors @jmduarte
• [x] Document extension API @jmduarte
• [x] Document FIFO buffer depth optimization in flows.rst @jmduarte
• [x] Document available backends in status.rst @jmduarte
• [x] Choose name for v0.7.0 @thesps (how about delphinium?)
• [x] Document optimizers and flows in flows.rst @jmitrevs
• [x] Document backends and I/O in details.rst @jmitrevs
• [ ] Document VivadoAccelerator with link to tutorial @jmduarte
• [x] Clarify level of support for Keras/PyTorch/(Q)ONNX in status.rst @jmitrevs
• [ ] Improve docstrings / autogenerated docs @vloncar

[jmitrevs]

The documentation mentions to check out example-models, but it's a submodule, so I would recommend we suggest the submodule way of doing it. Then it's also in the right place.