Add developer documentation

[alanking]

• Deprecation/end of life (see Document deprecation rules #124)
• Development environment (see Developer Documentation: Using the development environment #219)
• Building and installing iRODS (Developer Documentation: Building and installing iRODS #220)
• Onboarding new developers (Add documentation for "getting started" for new iRODS developers #221)
• Testing environment (Developer Documentation: Using the testing environment #222)
• How to use Github
• How to use rr/debugging tools (Developer Documentation: Debugging iRODS (using rr/gdb) #223)
• Valgrind (Developer Documentation: How to track down memory leaks (using Valgrind, memcheck, ASAN, etc.) #224)
• Static analysis (Clang) (Developer Documentation: Using static analyzers (clang-tidy, clang-format, etc.) #225)
• Running (and adding) tests (Developer Documentation: Adding and running tests #226)

[alanking]

Could probably use some documentation around plugin operations as well as instructions for generic (and specific) plugin implementation.

[korydraughn]

this should include documentation on coding conventions, for example, the JSON property name for instructing the server to execute the operation as an administrator should be admin_mode and accept a boolean value.

[trel]

best practices section...

• should include target library advice / do and do nots.
  • Object library target irods_plugin_dependencies_functions has public non-target dependency irods#6218 (comment)

[korydraughn]

add notes to the developer docs about invoking external programs from C++ code.

from irods/irods#6272 (comment)

[alanking]

I split up all the tasks into separate issues. Closing.