Review and update the /customization doc pages to adhere to SSOT standards

Following the instructions in &1280 (closed):

1. review the following docs, updating if needed.
2. log your findings/work in the sheet in realtime as you work.
3. where you needed to make improvements to files, summarize those changes in the MR descriptions.
4. add the MR links to the sheet and link them to this issue. (One MR can affect multiple files, where appropriate.)

Pages included:

customization/index.md
customization/branded_login_page.md
customization/branded_page_and_email_header.md
customization/favicon.md
customization/help_message.md
customization/libravatar.md
customization/new_project_page.md