a11y: adopt pa11y wcag2aaa preset

[SethFalco]

Switches the standard of Pa11y we follow from WCAG2AA to WCAG2AAA. In general, I only aim for AA criteria, but for the rules that can be checked automatically, I'd like to strive for AAA as well.

Most of the issues reported by Pa11y were color contrast issues on light mode.

1. Mostly because I don't use the site in light mode, so I clearly wasn't paying enough attention.
2. The default Docusaurus theme only strived for WCAG A or AA, not AAA. So we have to override more of the defaults to push it over to the next level.

Changes

	Before	After
Links
Sidebar (Light mode)
Preset Badge
Footer

In the screenshots above, the differences probably look pretty subtle. However, when actually using the site it's more apparent.

Chores

Color Model

While doing this, I also migrated most colors from hex to HSL. During development, I always use HSL anyway, I just persisted them as hex out of habit. However, HSL is much more intuitive to work with, and the colors are replaced with hex during the build anyway.

!important

When overriding a CSS variable from a theme/plugin, I'm just defaulting to use !important now.

Historically, I only ever used !important when it seemed necessary, but now I find it be simpler when working with third-party variables.

Related

• Pa11y originally added in chore: add a11y step to ci for wcag aa guidelines #54