chore(docs): installing-superset-from-scratch guide gets complete, orderly

[AlphaJack]

Now guide can be followed directly: venv is the only Python environment mentioned, needed environmental variables are stored on a file and sourced from there. All superset commands can now be run without interruptions

SUMMARY

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API

[sfirke]

This looks like a wonderful PR! People get tripped up by these steps and the way you reordered and added necessary config variables should help a lot. It looks good to me, but I have never actually installed Superset from scratch (I only have deployed with Docker) so don't feel qualified to merge it myself. Hopefully someone else can take a quick peek, agrees with me, and we can merge ASAP.

[AlphaJack]

@sfirke Based on the "Installing from Scratch" guide I created superset-venv, an AUR package for Arch Linux users.

Unfortunately it was not possible to create a non-venv superset, as the app requires specific dependency versions, either too old or not yet available from Arch repos.

In both packages I included a message with the manual steps needed to finalize the installation.