JITServer AOT cache developer documentation

[AlexeyKhrabrov]

JITServer developer documentation under https://github.com/eclipse-openj9/openj9/tree/master/doc/compiler/jitserver currently has no information about the AOT cache implementation. Most of the things that should be documented are already described in the issue #12153 and the pull requests linked from there (and their commit messages). All that information needs to be put together and organized as part of JITServer developer docs.

[mpirvu]

@AlexeyKhrabrov could you please review eclipse-openj9/openj9-docs#961
This is user documentation for JITServer AOT cache feature. Thanks