Move code of conduct and discord guidelines docs into the website

[fatmahussein]

Summary:
This PR introduces Markdown page support for the WNB.rb website including styling, routing and behavior improvements for external links. It replaces Google Doc redirects for the Code of Conduct and Discord Guidelines with self-hosted Markdown pages.

Changes Include

1. PagesController

• Added PagesController#show to render Markdown files from app/content.
• Introduced HTMLWithBlankLinks class so all Markdown links open in a new tab.
• Uses Redcarpet gem for Markdown rendering with safe HTML filtering.

2. Routing

• Updated /code-of-conduct and /discord-guidelines routes to use the Markdown pages.
• Defaults now point to respective Markdown files (code_of_conduct.md, discord_guidelines.md).

3. Markdown Styling

• Added markdown.scss to style headers, paragraphs, lists, images and links.