Support for notes referenced more than once and for endnotes in markdown (plus DOCX and ODT)

[massifrg]

I pushed a commit that provides support in Markdown for:

• multiple references to the same note (see Markdown footnotes are duplicated #1603)
• endnotes (see Allow multiple note types (footnotes, endnotes, etc.) #1425)

This is achieved:

• without changing current pandoc types
• without changing Markdown's syntax
• via extensions that are disabled by default, so nothing changes if you don't enable them

I'm providing the documentation here.

Support for endnotes includes the commits from #11501, #11508, #11516.