feat: add the "view page source" link to pages

Description

I think we had this at some point (it's referenced in #62), but it's no longer visible. According to https://squidfunk.github.io/mkdocs-material/setup/adding-a-git-repository/#code-actions you need to add the edit or view actions to the theme features.

When I tested this, both the edit and view actions linked to the same gitlab page (the view preview of markdown doc view), so I kept just the view action.

Supporting information

Private-ref: BB-10597

Ref: #62

Testing instructions

• build the site (or view the preview environment link)
• visit a few pages
• verify each page has a "view source" icon
• verify that clicking this icon navigates to the correct corresponding page of the handbook in gitlab