doc: add field 'Constraints' to 'boards.yml'

[umarcor]

This is a follow-up of #160, related to #105.

An optional field, Constraints, is added to each item in boards.yml. Constraints can contain an string or a list of strings, matching the boad identifiers used in hdl/constraints.

At the same time, an intersphinx mapping to hdl/constraints was added to the documentation configuration. When building the documentation, a new column is added to the boards compatibility table, which contains cross-references to hdl.github.io/constraints/Data/Boards. See umarcor.github.io/openFPGALoader/compatibility/board.

@trabucayre, as discussed in #105 (comment), the purpose is each repo to keep their own data/details. These "keywords" allow cross-referencing through interpshinx, as shown in this PR. Furthermore, after this PR is merged, we can use the boards.yml and boards.py files from this repo to enhance the documentation of hdl/constraints.

We can later improve this iteratively:

• Add misssing boards to hdl/constraints.
• Evaluate adding fields to the YAML here or there, and/or moving fields from one to the other.
• Update the layout in the site here, in order to show a section per board, instead of or apart from the table.
• etc.

[trabucayre]

Really useful informations/field!
I have to see how to limit columns size to avoid a big horizontal scroll.
Thanks!
Applied