Revert "Bump html-pipeline from 2.5.0 to 2.11.0"
Revert "Bump html-pipeline from 2.5.0 to 2.11.0"

This reverts commit 450edfae1c3202b19587d34acc5463347c6158aa.

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'

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs