Build output serial number docs

[SchrodingersGat]

• Improve documentation around serial numbers for build outputs
• Allow build outputs to be created manually without serial numbers
• Brings functionality into line with external build orders (where serials are not required)

[netlify]

✅ Deploy Preview for inventree-web-pui-preview canceled.

Name	Link
🔨 Latest commit	9179d92
🔍 Latest deploy log	https://app.netlify.com/projects/inventree-web-pui-preview/deploys/6a6858e89c93350008039be6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.76%. Comparing base (4a8ee54) to head (9179d92).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12486      +/-   ##
==========================================
- Coverage   86.79%   85.76%   -1.04%     
==========================================
  Files        1443     1444       +1     
  Lines       95858    97308    +1450     
  Branches    11214    11214              
==========================================
+ Hits        83203    83457     +254     
- Misses      12591    13787    +1196     
  Partials       64       64              

Flag	Coverage Δ
backend	90.75% <100.00%> (-0.22%)	⬇️

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Components	Coverage Δ
Backend Apps	90.22% <100.00%> (-2.22%)	⬇️
Backend General	93.53% <ø> (ø)
Frontend	79.53% <ø> (ø)

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