Document which table features are supported in which formats

[jgm]

Now that the new col/rowspan and other fancy table features are getting implemented in some formats, it would be good to have a place where we document which features work in which formats. (A table, maybe?)

[jgm]

Just looking through the git log, I'm seeing this for changes since the last release:

docbook reader:

• table cell alignment DocBook reader: Implement table cell alignment #6698
• colspan Reference Not Found Error #6592

latex reader:

• rowspan, colspan LaTeX reader - support new table features #6311

opendocument writer:

• cell alignment OpenDocument Writer: Implement table cell alignment #6700
• row/colspan OpenDocument writer: New table cell support with column spans #6682

html writer:

• intermediate table headers HTML writer - support new table features #6314
• table footers HTML writer - support new table features #6314
• col/rowspans HTML reader - support new table features #6312