add design motivations + related best practices section(s) to documentation

[jrevels]

xref #54 (comment)

• https://gist.github.com/jrevels/fdfe939109bee23566d425440b7c759e
• data production/consumption to/from schema-on-read systems
• data production/consumption to/from (Legolas-aware) schema-on-write systems (esp. the notion of "sealing"/"extensibility", i.e. whether or not the system supports registration of extensions of its supported schema versions)
• Legolas as a mini-IDL that encodes "be liberal in what you consume, conservative in what you produce" in its semantics
• theoretical underpinnings w.r.t. lenses/optics; "optical data systems"
• comparison to other system's "record types"
  • https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/record
  • https://garywoodfine.com/c-records-the-good-bad-ugly/
  • https://dev.realworldocaml.org/records.html