Resolve "Mermaid diagrams and charts are not visible in the login page"

What does this MR do and why?

Enables GitLab Flavoured Markdown - Diagrams and Flowcharts rendering in the login page.

This MR fixes the issue #425011 (closed).

Applied renderGFM(document.body) in the following pages:

• sessions:new (/users/sign_in)
• registrations:new (/users/sign_up)
• passwords:new (/users/password/new)
• groups:sso:saml (for example, /groups/flightjs/-/saml/sso)
• admin:application_settings:appearances:preview_sign_in (/admin/application_settings/appearance/preview_sign_in)

Screenshots or screen recordings

Login Page

Before	After

Preview Sign-in Page

Before	After

SAML single sign-on

Before	After

How to set up and validate locally

Run GDK locally and set the appearance of the sign-in pages:

• [1] Admin > Settings > Appearance > Sign in/Sign up pages > Description
• [2] Admin > Settings > Preferences > Sign-in and Help page > Additional text to show on the sign-in page
• [3] Admin > Settings > General > Sign-in restriction > Sign-in text

Use the following content to fill the text fields:

Test MR 133524.

```mermaid
graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;
```

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #425011 (closed).