Inconsistencies with custom renderers in the Static Site Editor

Problem to solve

In the process of applying a consistent Markdown format to the Product pages of the GitLab handbook, the following bugs and inconsistencies with our custom renderers for displaying non-markdown content in the WYSIWYG editor were found:

Rendering identifiers

1. https://about.gitlab.com/handbook/product/product-categories/plan/
   • This page does not load any data into the Static Site Editor
   • Fixed: gitlab-org/gitlab#232566 (comment 387678825)

Rendering ERB blocks

1. https://about.gitlab.com/handbook/product/investment/index.html.md.erb
   • Line 30 marks the beginning of some Ruby code in a table. This doesn't get handled correctly in the WYSIWYG editor. See Design file 1 - investment
   • Fixed: !57748 (merged)
2. https://about.gitlab.com/handbook/product/product-categories/ops/
   • Loading this page in the SSE incorrectly displays the first block of Ruby (see 2 - ops) and the resulting MR did not apply any of the expected formatting changes
   • Fixed: !57761 (merged)
3. https://about.gitlab.com/handbook/product/application-types/
   • The ruby seems to have been a little garbled in this one MR, see line 40 of the current file, there's some text escaping that's being removed.
   • NOT fixed: see #8343 (comment 387513763)
   • Fixed (with new approach): see gitlab-org/gitlab#232567 (closed)

Rendering / formatting lists

1. https://about.gitlab.com/handbook/product/product-principles/

   • This MR was the result but it didn't correctly translate the spacing for the nested list items under the Our Product Principles header. I added the spaces back in manually to merge it, but we should investigate why it didn't pick up the intended indent level.
     • formatting anti-pattern: Fix in the page’s source.

2. https://about.gitlab.com/handbook/product/single-application/

   • This MR shows a weird issue where ordered lists with 10+ items start getting numbered sequentially. Fixed the formatting manually in this commit
   • Also happened on https://about.gitlab.com/handbook/product/product-management/walk-throughs/ops-section/ so it's pretty consistent
     • Fixed

Formatting strong / emphasis text

1. https://about.gitlab.com/handbook/product/how-to-engage/
   • On line 100, something weird happened where the SSE moved the _ to a different spot in the sentence. Fixed the formatting manually in this commit
     • Fixed