Update just-the-docs requirement from 0.7.0 to 0.8.1

Updates the requirements on [just-the-docs](https://github.com/just-the-docs/just-the-docs) to permit the latest version.
- [Release notes](https://github.com/just-the-docs/just-the-docs/releases)
- [Changelog](https://github.com/just-the-docs/just-the-docs/blob/main/CHANGELOG.md)
- [Commits](just-the-docs/just-the-docs@v0.7.0...v0.8.1)

---
updated-dependencies:
- dependency-name: just-the-docs
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gem "jekyll", "~> 4.3.2" # installed by `gem jekyll`
 gem "webrick"          # required when using Ruby >= 3
 
-gem "just-the-docs", "0.7.0"
+gem "just-the-docs", "0.8.1"
 gem "jekyll-relative-links"
 gem "jekyll-sitemap"
 gem "jekyll-seo-tag"