[Docs update] Manual installation steps minor edits

[kdmukai]

Description

• Adds additional time guidance for long-running setup tasks.
• Removes the virtualenv

Rationale: virtualenvs are only useful when multiple python projects run on the same machine. Instead we just directly install pip dependencies into the installed python3.

This pull request is categorized as a:

• [x] Documentation

Checklist

If you modified or added functionality/workflow, did you add new unit tests?

• [x] N/A

[jdlcdl]

I like less to do w/o virtualenv, besides it's sort of a preference that devs will know to do, and how they prefer to do it if they're aware of it.

Question though: were any "sudo"s necessary that might not have made it into these docs changes?

[kdmukai]

@jdlcdl The only altered commands are the python3 -m pip calls instead of running pip from within the virtualenv, but those do not require sudo (in general sudo + pip only creates trouble).

[jdlcdl]

ACK

This worked for me today... but I always removed the recently suggested /swapfile as root (I think to build numpy w/ less than 1gb ram)

[newtonick]

ACK, these changes worked in my brief testing.