Make docs site (jrnl.sh) fully meet Web Content Accessibility Guidelines (WCAG) 2.1 (#1105)

* add some attribtutes to docs template for accessbility

* fix colors to meet accessibility guidelines (4.5 contrast ratio for text)

* Fix last remaining pa11y error (no button on search form)

This fix required moving the mkdocs theme out of the docs directory.
It's no in the docs_theme directory, and the mkdocs config is updated
accordingly.

* Re-enable accessibility testing for docs sit

Also, move the pa11y script into the gh actions workflow

* clean up linting issues in css

* fix and standardize link colors across site

* fix twitter button opacity making text fail contrast requirements

* move buttons on docs site index nav, tweak font weights

* fix footer opacity, tweak spacing of the now more visible sections of the page

* change font sizes on index page to meet WCAG

* udpate font sizes site-wide for accessibility

* fix sidebar for accessibility (font sizes and color contrasts)

* restyle code blocks to have dark background, and meet accessibility requirements

* standardize (accessible) colors across docs site

docs_theme/assets/colors.css

@@ -0,0 +1,28 @@
+:root {
+  /* For dark bg */
+  --white: #fcfcfc;
+  --off-white: #f4f0ff;
+  --purple: #7e57c2;
+  --light-purple: #cf93e6;
+  --blue: #61aeee;
+  --green: #a6e22e;
+  --orange: #fd971f;
+  --red: #eb5567;
+  --pink: #d57699;
+  --yellow: #e2b93d;
+
+  /* For light bg */
+  --teal: #2a8068;
+  --dark-blue: #356eb7;
+  --mid-purple: #92679b;
+  --bright-purple: #af27ad;
+  --dark-purple: #604385;
+  --darkest-purple: #251A32;
+  --grey: #3b3b4a;
+
+  --black-shadow: #0000001A;
+  --blacker-shadow: #00000059;
+
+  /* Special cases */
+  --terminal: #1b1c2e;
+}