Update installation docs with typo and consistency fixes

[jacobtylerwalls]

brief description of changes

• Fix typos
• myproject -> my_project per previous steps of tutorial
• Use full path to requirements.txt
• Link to stable versions of Django docs (so that we are not continually updating 1.11 -> 2.x -> 3.x etc.)
• Update url() examples to use path() (deprecated in Django 3.1)
• Suggest Python 3.8 as minimum (due to importlib_metadata issue)

issues addressed

Closes #356

This box must be checked

• the PR branch was originally made from the base branch

This box should be checked

• after these changes the docs build locally without error (there are unrelated errors before my changes...)

This box should only be checked you intend to follow through on it (we can do it on our end too)

• I will cherry-pick all commits in this PR into other branches that should have them after this PR is merged

[ekansa]

@jacobtylerwalls this looks good to me! Thanks for the updates. I think they are very helpful!!

[jacobtylerwalls]

Glad to hear it!