Developer documentation

[John Hendrikx]

In PR https://github.com/openjdk/jfx/pull/910 a lot of "new" insights 
were gained in the snapping logic.  Michael Strauss suggested 
documenting this, and I thought we may as well discuss this on the 
mailing list instead of continuing the discussion in that PR.

In my normal line of work, I usually encourage projects to include 
developer documentation as part of the Git repository. This allows any 
developer to access and modify the documentation easily, and allows you 
to keep documentation in sync with relevant commits (and one can ask 
developers to do so as part of the PR).

The documentation is provided in markdown format and is usually stored 
in a /doc folder. If there are multiple modules, there can be 
specialized doc folders per module with a top level doc folder which 
contains an index.md linking all the docs together, and containing 
documentation that is not module specific.

The documentation is intended for developers only, not for users of 
JavaFX, and hence does not need to be published. Markdown files can 
either be read directly in your IDE of choice or online via GitHub/GitLab.

The build documentation may be a good candidate to place there as well.

So, my suggestion would be:

- Create a top level /doc folder, and create module level /doc folders 
as needed when relevant documentation is written
- In each /doc folder there is an index.md file that links to all 
documentation in that folder
- A higher level index.md also contains links to child indexes
- Consider moving the build and any onboarding documentation there
- The top level README.md should have a link to /doc/index.md
- Use only GitHub supported markdown features

--John