docs: Reorder install instructions in CONTRIBUTING.md

[willbarrett]

SUMMARY

The install documentation recommended creating an admin user before setting up the database. This PR reorders the install instructions so that the database structure is created before attempting to create an admin user.

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

[willbarrett]

Pinging @dpgaspar for a review because it's a little bit FAB-related, and I miss his face.

[codecov]

Codecov Report

Merging #17387 (ea5c576) into master (7d22c9c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #17387   +/-   ##
=======================================
  Coverage   77.02%   77.02%           
=======================================
  Files        1038     1038           
  Lines       56013    56013           
  Branches     7735     7735           
=======================================
  Hits        43142    43142           
  Misses      12613    12613           
  Partials      258      258           

Flag	Coverage Δ
hive	81.48% <ø> (ø)
javascript	71.22% <ø> (ø)
mysql	81.90% <ø> (ø)
postgres	81.91% <ø> (ø)
presto	81.78% <ø> (ø)
python	82.41% <ø> (ø)
sqlite	81.59% <ø> (ø)

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[srinify]

I'll approve this :) I spent a bunch of time recently with the first "one minute" of setting up superset and this change makes sense. Thanks Will!

[villebro]

LGTM, thanks for the improvement!

[dpgaspar]

Great to "see" you here @willbarrett! this makes perfect sense.