Revert "Bump html-pipeline from 2.5.0 to 2.11.0"

This reverts commit 450edfa.

This bump is causing issues with ASCII/UTF-8 processing which is
preventing the docs from building. Likely to be a change from
`github-markdown` to CommonMark. Reverting before more investigation.

Error processing resource for index: apis/search/faceted-search.html
"\xE2" from ASCII-8BIT to UTF-8
 govuk-developer-docs/.bundle/gems/commonmarker-0.19.0/lib/commonmarker.rb:26:in `encode'

Gemfile.lock

@@ -91,7 +91,7 @@ GEM
       concurrent-ruby (~> 1.0)
     hashdiff (0.3.7)
     hashie (3.6.0)
-    html-pipeline (2.11.0)
+    html-pipeline (2.5.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
     http_parser.rb (0.6.0)