build: bump html-proofer from 3.11.1 to 3.12.0 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.11.1 to 3.12.0.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.11.1...v3.12.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

docs/Gemfile.lock

@@ -84,13 +84,13 @@ GEM
     html-pipeline (2.12.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.11.1)
-      activesupport (>= 4.2, < 6.0)
+    html-proofer (3.12.0)
       addressable (~> 2.3)
-      mercenary (~> 0.3.2)
-      nokogiri (~> 1.9)
+      mercenary (~> 0.3)
+      nokogiri (~> 1.10)
       parallel (~> 1.3)
       rainbow (~> 3.0)
+      timerizer (~> 0.3)
       typhoeus (~> 1.3)
       yell (~> 2.0)
     http_parser.rb (0.6.0)
@@ -244,6 +244,7 @@ GEM
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     thread_safe (0.3.6)
+    timerizer (0.3.2)
     typhoeus (1.3.1)
       ethon (>= 0.9.0)
     tzinfo (1.2.5)