Documentation needs to be updated to 1.6.1

[ethangreen-dev]

Our documentation is currently pinned to release 1.2.0 as seen here. This needs to updated to the latest release.

[ethangreen-dev]

Since documentation depends on the contents of the docs directory in the main branch, we will need to update documentation prior to merging into main.

[ethangreen-dev]

We need to determine where the documentation will live. If we continue to use readthedocs.io as our host a readthedocs branch would be ideal. If we use Github Pages, then we need to push changes to gh-pages. This decision is highly dependent on whether we can get access to configure the webhook that powers gillespy2.readthedocs.io.

Due to the scope of this change #494 will be resolved alongside this issue.

[ethangreen-dev]

In addition -- if we change our documentation host then all references to previous sites will need to be updated.