[WIP] changes for next release 3.10.3

[nlohmann]

This thread collects all changes made since the 3.10.2 (#2978) release.

Improvements

• Allow to write binary formats to vectors with custom allocators.
  • to_{binary format} does not provide a mechanism for specifying a custom allocator for the returned type. #2982
  • Allow allocators for output_vector_adapter #2989
• Allow conversion from types that do not define an explicit iterator type, but have a begin() and end() function. By @theodelrieu
  • meta: fix is_compatible/constructible traits #3020

Bug fixes

• Fix bug in update function when used with extended diagnostics. By @AnthonyVH
  • Parent pointer not properly set when using update() #3007
  • Set parent pointers for values inserted via update() (fixes #3007). #3008
• Fix bug when inserting into arrays when using extended diagnostics. By @carlsmedstad
  • Yet another assertion failure when inserting into arrays with JSON_DIAGNOSTICS set #3032
  • Assertion failure when serializing array with JSON_DIAGNOSTICS set #2926
  • Fix assertion failure for JSON_DIAGNOSTICS #3037

Further Changes

• Update Docker image.
  • Use new Docker image #2981
  • Re-add Clang 12 #2986
• Add script to generate docset for Dash, Velocity, and Zeal.
  • Update docset generation script #2967
• Fix compiler version mentioned in the README file.
  • Fix Clang version #3040