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

[wren]

Fixes #1061

This PR fixes all of the acessibility issues that pa11y was reporting on our docs site. It also re-enables accessibility testing for PRs and branches that change anything related to the docs.

The main issues were:

1. Low contrast colors choices
   jrnl.sh will now meet the AAA standard specified in WCAG 2.1, which is to have a minimum of 4.5:1 contrast ratio for all text on the site.
2. Our search bar was missing a submit button
   No submit button makes it hard for anyone that can't use a keyboard to search our docs site. Now there's a button, so everybody wins.

Note that I had to move the mkdocs theme folder out of docs and into docs_theme. This is because readthedocs doesn't recognize main.html (which is necessary to add the missing search button) inside of the docs directory. Mkdocs' site says not to put the custom theme inside the docs directory anyway, so I guess this fixes both of those issues at once.

Also, here's an example build where this all ran successfully:

• https://github.com/wren/jrnl/runs/1502555163?check_suite_focus=true

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.
• All tests pass.

[wren]

I tried to keep the colors as similar to the original as possible, but now with the higher contrast, some of the colors look a bit garish.

Like this page honestly looks a bit alarming. Maybe we should tone down the red?

[wren]

Okay, all colors are now updated, and this PR should be good to go.

[micahellison]

📄📄📄📄

[github-actions]

Unit Test Results

  9 files  9 suites   0s ⏱️
49 tests 0 ✔️ 0 💤 49 ❌

results for commit 558d331