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

mkdocs.yml

@@ -2,11 +2,12 @@ site_name: jrnl
 site_url: https://jrnl.sh
 theme:
   name: readthedocs
-  custom_dir: docs/theme
+  custom_dir: docs_theme
   static_templates:
       - index.html
 extra_css:
     - https://fonts.googleapis.com/css?family=Open+Sans:300,600
+    - assets/colors.css
     - assets/theme.css
     - assets/highlight.css
 markdown_extensions: