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>

fixtures/html5validator-config.yml

@@ -1,6 +1,3 @@
 root: _site
 blacklist:
   - "line-numbers"
-ignore:
-  - "1.810-1.823: error: CSS: Parse Error."
-  - "1.811-1.824: error: CSS: Parse Error."