Improve docs related to LMS/CMS testing

[robrap]

A/C

• Update devstack testing doc to point to the correct readthedocs version: https://docs.openedx.org/projects/edx-platform/en/latest/concepts/testing/testing.html
• Remove redundant or outdated information

Original ticket:

1. It isn't clear from these docs that a browser container is needed when running the JS tests. It would be useful to mention to note that.
2. The link to the edx-platform testing doc is broken. It should point to readthedocs version here: https://docs.openedx.org/projects/edx-platform/en/latest/concepts/testing/testing.html
3. Related to the above, the edx-platform docs already includes notes about devstack, and requiring the browser container here: https://docs.openedx.org/projects/edx-platform/en/latest/concepts/testing/testing.html#running-javascript-unit-tests
4. The list of commands in the devstack doc is outdated, since we don't have bokchoy (and a11y?) tests. Can we remove most of this redundancy and just point to the edx-platform testing doc instead?

Other related notes:

6. The ./in hack is probably more globally useful (maybe?)? Could this be described elsewhere more generally, and maybe just linked to here? I didn't know this script existed.
7. Should make lms-shell be updated to make dev.shell.lms? Do we have a preferred standard, or do we want the docs to use assorted versions of our commands?