[MSITE-1036] support shared source directory instead of format-specific ones

**[Hervé Boutemy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hboutemy)** opened **[MSITE-1036](https://issues.apache.org/jira/browse/MSITE-1036?redirect=false)** and commented

src/site/\/ source content *.\
makes it hard to maintain content in a directory with multiple formats

supporting src/site/source content *.\
could ease maintenance

we need to define what to do with site.xml
or when multiple formats share same extension

---

**Issue Links:**
- [DOXIASITETOOLS-363](https://issues.apache.org/jira/browse/DOXIASITETOOLS-363) support shared directory for multiple markup formats
(_**"requires"**_)

Start by reading the linked DOXIASITETOOLS-363 issue and tracing how src/site// source content and site.xml are currently handled. Define the expected behavior for a shared src/site/source directory, including site.xml and cases where multiple formats use the same extension; done requires that behavior to be agreed before implementation and validation.

Written by the indexing model from the issue text.