[Documentation]: point users to a specific version of RMG rather than latest

[JacksonBurns]

TL;DR: This PR changes a couple lines in the docs to tell users to download RMG v3.1.1, which is a temporary release we decided on in RMG subgroup to point users toward for RMG training (rather than latest).

Right now the documentation points users towards the latest build, which is the one that is constructed every night and after every merge to main. We should actually point them toward some specific version, which will allow us to actually debug issues since everyone who has problems will be running a version we can try ourselves. The latest build should be used by devs, or in the case that an error has been fixed but a minor or patch release has not yet been published. This should consequently include a more consistent release schedule.

To coincide with this I have pushed a 3.1.1 version of RMG to docker. This is not intended for long term use but is instead a stop-gap solution. We already have a PR open for 3.2 #1852 but that will not be done in time for RMG training. This patch release includes all changes from 3.1.0 to today (which will also be in 3.2, and this patch will be deleted).

[codecov]

Codecov Report

Merging #2432 (3c4e8f9) into main (dd638de) will not change coverage.
The diff coverage is n/a.

❗ Current head 3c4e8f9 differs from pull request most recent head 95f82cd. Consider uploading reports for the commit 95f82cd to get more accurate results

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[hwpang]

This looks good to me! Only two things:
I see that you merge main into this branch, could you rebase this branch onto main instead?
I have this PR #2434 opened up to add several jupyter notebooks for RMG training, and we should make sure that the eventual docker 3.1.1 we tag contains this PR.

[hwpang]

Gonna force merge this to be in time of tomorrow's RMG Training as well.