Add what's new for v0.7.0 (#450)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Since v0.7.0 makes many changes, I've added a separate page documenting
all the new features, similar to what was done in v0.5.0.

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [N/A] I have added a one-line description of my change to the
changelog in
      `HISTORY.md`
- [x] This PR is ready to go