Updates to 7.4 installation instructions

[jacobtylerwalls]

describe the issue

I worked through a fresh arches installation following the docs, which were very helpful. I took note of updates that would be worth making along the way. I plan to submit a PR with commits that can be cherry-picked to necessary branches.

please add links to existing docs or code (if relevant)

• typos, broken links (various)
• yarn install incantation vs yarn build_development incantation (believe they should both be yarn build_development?)
• Arches no longer installs cleanly on Python 3.7 due to an importlib_metadata issue, so suggest starting on Python 3.8+
• Update url() examples to use path() (deprecated in Django 3.1)

which release does this issue concern?

stable 7.4