Take a pass through docs before cutting release

[chiphogg]

I'm guessing we'll have missed a bunch of stuff, and we definitely want to reflect it in the released version of the doc website. Including, but not limited to:

• Troubleshooting guide
• Add upgrade guide how-to doc
  • Explain future proofing
• Alternatives
  • Lib versions
    • Boost units: version is 1.2
    • nholthaus units 2.3.4
    • bernedom/si 2.5.4
    • mp-units 2.4.0
  • Ease of Acquisition
    • Au Available on all platforms
  • Features
    • Conversion safety
      • Separate risk checks for overflow and truncation
    • Point types
      • Efficient common units
    • New: Negative units
    • mp-units point and delta utilities
• New conversion risk docs
  • Reference: conversion risk policies page
  • Discussion: concepts:
    • new "conversion risks" page
    • new "truncation" page
  • Updates for reference pages
    • constant/
      • Document policy APIs
      • "Soft deprecate" coerce APIs
    • quantity/
      • Document policy APIs
      • "Soft deprecate" coerce APIs
      • Update discussion around "explicit Rep"
  • quantity_point
    • Document policy APIs
    • "Soft deprecate" coerce APIs
    • Update discussion around "explicit Rep"
  • tutorial/
    • 103-unit-conversions/
      • Update lossy conversions section, and Takeaways
      • Update exercise 1(b)
• Common point unit
  • discussion/concepts/
    • common unit
      • common point unit is common unit of units, and origin displacements
      • include example of Celsius and Kelvins
  • reference/
    • unit/
      • Common point unit is common unit of units, and origin displacements
• Misc
  • discussion/
    • concepts/
      • dimensionless
        • Add "as raw number" section
        • Include policy APIs
      • overflow
        • Update reference to 110
    • implementation/
      • applying_magnitudes/
        • Update text around "impossible" situations
  • reference/
    • math/
      • hypot
    • quantity_point
      • Fix default value implementation and docs

Checkmarks mean "I wrote this locally".