Add support for tables

[MisterDA]

In documentation it can be really handy to have tables (for instance, for side-by-side comparison). It would be nice if odoc had a native syntax support for tables. For now I resort to using the {%html: …%} tag.

[jonludlam]

@gpetiot has made a draft PR with a suggestion for concrete syntax on the odoc-parser repository - I think it would be very useful to get some feedback there.

[emillon]

Done in #893 if I'm not mistaken

[panglesd]

Yes!