Change installation instructions to suggest installing from pypi instead of git clone

[garrison]

At some point, we will likely want to change the installation instructions to suggest that users install the latest stable version via pip install from pypi rather than the latest development version (main branch) via git clone.

A few open questions, though:

1. When is the timing best for making this change?
2. How do we anticipate the user will obtain the correct tutorial notebooks? (They won't be provided by any pip install command.)
3. Do we want to update the Dockerfile, too, so that it installs the latest released version?
4. Should we, concurrently with this change, make it so the latest stable version's documentation is hosted on the web?

[garrison]

(1) is addressed by #213, and we are punting on (2). We should plan to revisit (3) and (4) soon.

[garrison]

We now deploy docs from the stable branch, so (4) is complete.

[garrison]

I think everything actionable here is done. For (2), if somebody wants the Dockerfile to use a released version, they should check out that version's tag first.