Add TOC to policy pages

[remibetin]

This PR displays a table of contents on country pages.

Preview in context (from wai-policies-prototype repo)

The implementation differs from the exploratory PR in some ways:

• The TOC is positioned after the last updated date. I find this cleaner.
• The TOC is added to the current layout in wai-website-theme, used in production. As such, it does not support states and provinces. I suggest to open a distinct PR when the state/province level policies will be added to the pages.
• The TOC uses the toc.html include for consistency.

This PR also:

• makes the disclaimer at the bottom of a country page more prominent, by including it into a box.
• uses h2s instead of boxes to delimit each policy on a country page, as in the exploratory PR.
• stops passing an id parameter when calling multilang-list-policy-links.html include. This parameter was not used by the include file.

[remibetin]

@kfranqueiro @iadawn FYI in case you have started looking at this PR, I have just made a few changes to the PR (0602eae and 144659d) and updated the PR description accordingly.