Make navigation and related-page tiles feel more clickable

[bedroesb]

Closes #426.

This PR improves the clickable affordance of section navigation tiles and Related pages tiles by adding a consistent arrow cue and subtle hover/focus animation.

Changes include:

• Add a shared navigation-tile-card.html include for section navigation tiles and related pages.
• Add a visible arrow affordance to clickable navigation tiles, with hover/focus movement and no hover shadow.
• Simplify simple section tiles with left-aligned titles, cleaner title fitting, and a vertically centered arrow.
• Add custom support to section-navigation-tiles-simple.html.
• Update filter clear buttons to use round primary icon styling in line with the back-to-top button.
• Improve website section docs for type, custom, except, sort, and simple tiles.
• Clean up rendered docs snippets so include/tool examples no longer show extra blank space above and below.

New SASS variables

/*-----Section navigation tiles-----*/
$nav-card-arrow-bg: rgba($primary, 0.08) !default;
$nav-card-arrow-bg-hover: $primary !default;
$nav-card-arrow-color: $primary !default;
$nav-card-arrow-color-hover: $white !default;