Fix invalid CSS rules in head elements (#1368)

Rectify and simplify CSS rules  used in the head element. The rules now avoid the invalid use of nested `:not()` pseudo-classes.

- Update CHANGELOG.md
- Remove ignore statements re CSS parse errors
- Add rules for excluded pages and simplify
- Add `activation_no_nav_link` with the rules for excluded pages
- Insert `activation_no_nav_link` when no other rules are generated
- Replace `:nth-child(1)` by `:first-child`
- Correct `:nth-child(n + {{ activation_index | plus: 1 }})` to `:not(:first-child)`
- Eliminate `activation_collection_index`
- Generate `.nav-category-list` selectors only for sites with collections

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Co-authored-by: Matt Wang <matt@matthewwang.me>

assets/css/just-the-docs-head-nav.css

@@ -11,7 +11,7 @@
 
 {%- capture scss -%}
 {% include css/just-the-docs.scss.liquid color_scheme=color_scheme %}
-.nav-list .nav-list-item .nav-list-link {
+.site-nav ul li a {
   background-image: linear-gradient(
     -90deg,
     rgba($feedback-color, 1) 0%,