User guide layout (#5016)

- Modified CSS: font-size, colors, margins...

- Added an ArticleContent Component that will be used for the changelog
and user guide articles, which contains the styles for each titles,
paragraphs, images etc.

- Added link to User Guide in footer

- Added a UserGuideWarning Component: 

<img width="332" alt="Screenshot 2024-04-17 at 18 14 48"
src="https://github.com/twentyhq/twenty/assets/102751374/0f5c601b-a2c0-4c63-baeb-1990d65fc332">

- Added a UserGuideEditContent Component:

<img width="394" alt="Screenshot 2024-04-17 at 18 16 24"
src="https://github.com/twentyhq/twenty/assets/102751374/6c7c3bd4-c5bb-4d02-8f93-bd99bc30ce7b">

- Added a UserGuideLink Component (for usage in MDX to allow links to
open in new tab)

- Fixed table of content

- Made responsive

---------

Co-authored-by: Ady Beraud <a.beraud96@gmail.com>
Co-authored-by: Félix Malfait <felix.malfait@gmail.com>

packages/twenty-website/src/app/_components/user-guide/UserGuideSidebar.tsx

@@ -49,6 +49,7 @@ const StyledHeadingText = styled.div`
   cursor: pointer;
   font-size: ${Theme.font.size.sm};
   font-weight: ${Theme.font.weight.medium};
+  color: ${Theme.text.color.secondary};
 `;
 
 const UserGuideSidebar = () => {