ATRST-2: Manifest Builder^

Description^

Implement function that sets up an atDoc manifest. It should check if the cache is up to date and load that if not or if it is told to ignore the cache. If the cache is not up to date it should call a function to generate an at_document_t and parse the file as reStructuredText. It should perform this operation for all reStructuredText-based source and include files.

Note, separate the manifest builder from the parse operation, as eventually we want this to run over multiple source formats in parallel.