Is there something like javadoc or rdoc for documenting YAML files, so that we could extract it into HTML documentation? Ideally with markdown syntax.
As it appears in the comments to the question, generally speaking, all that is necessary for documenting YAML is to create a section of the YAML content devoted specifically to documentation or metadata.
The only noteworthy challenge is determining whether you want your documentation section to conform with the conventions of any of the various syntax-styles for documenting code (e.g., Doxygen, NaturalDocs, whatever).
There are various approaches to this problem domain. Alternate approaches are indicated in the "See also" section of this answer.
Creating a metadata section in YAML is very straightforward, you can do it simply by creating an inline string where you dump all your documentation as a single block.
## comments
## NOTE: YAML generally throws your comments away, so they are not very useful
## for round-trip metadata
meta: |
Here is all my documentation and metadata
blah blah blah.
data:
branch_one:
- caption: blah blah
date: blah blah
details: blah blah
- caption: blah two
date: blah blah
details: blah blah
branch_two:
[..]