More comfortable styles for custom wiki sidebar content

What does this MR do and why?

Currently, users can create a custom sidebar markdown file for wikis, but the rendered markdown is not styled to take into account the fact that this content is displayed in a narrow aside.

This MR applies specific styling to custom sidebar content so that it is more ergonomic.

• Reduce sizes of headings
• Reduce left-padding of lists

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch
2. Go to a project with a wiki in the GDK
3. Create or edit the custom sidebar content
4. Add the following content to the custom sidebar:

# Getting Started

* [Get to know Knowledge Group!](https://example.com)

## Important Folders and Pages

* [Backlog](https://example.com)
  * [Bugs](https://example.com)
* [Blogs](https://example.com)
  * [Pages Blogs](https://example.com)
  * [Wiki Blogs](https://example.com)
  * [GLQL Blogs](https://example.com)
* [Knowledge Group Roadmaps by Quarter](https://example.com)
  * [FY26Q1](https://example.com)
  * [FY25Q4](https://example.com)
* [Metrics](https://example.com)
  * [Editors](https://example.com)
  * [GLQL](https://example.com)
  * [Pages](https://example.com)
  * [Wiki](https://example.com)
  * [Data Review Workflow](https://example.com)
* [Misc Pages](https://example.com)
  * [Competitive Analysis and Research](https://example.com)
  * [Dogfooding](https://example.com)
  * [First Iteration of GLQL JTBD](https://example.com)
  * [Matthews Test Pages for Stuff](https://example.com)
  * [Micro Teams](https://example.com)
  * [Release Post Items](https://example.com)
  * [Weekly Updates](https://example.com)

### Other resources

- foo
- bar

#### About the team

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##### People

- Gorm Blormley
- Livermack Fringle
- Hererera Bonigo
- Rolk Demplon

##### Dogs

- Frinky
- Dewola
- Somp
- Feefee

###### Cute dogs

- Loila Bonon

#### Our mission

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5. Save the changes
6. You will see the custom sidebar content rendered in the sidebar, with the styles from this MR applied.

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