[openstreetmap/openstreetmap-website] Add dismiss button for kramdown directions (PR #6370)

[rkoeze]

### Description
Add ability to dismiss the kramdown directions on larger screens. This allows users to expand the `textarea` to be the full width of the page, providing a cleaner and more focused writing experience. See screenshots for reference. On smaller screens the dismiss button is not visible since the kramdown directions are moved to the "Help" tab.

### Todos
- [ ] Add tests

### Screenshots
Before dismissing:
<img width="870" height="474" alt="Screenshot 2025-08-30 at 10 21 21 PM" src="https://github.com/user-attachments/assets/735a4707-58cc-4716-907e-62a6de06721c" />
After dismissing:
<img width="870" height="526" alt="Screenshot 2025-08-30 at 10 27 11 PM" src="https://github.com/user-attachments/assets/ee528119-1a42-46d0-9b60-6dfce5dd744d" />

### How has this been tested?
I've manually tested the functionality while running the app locally. Will also add tests.

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6370

-- Commit Summary --

  * Add dismiss button for kramdown directions

-- File Changes --

    M app/assets/javascripts/richtext.js (6)
    M app/views/shared/_richtext_field.html.erb (3)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6370.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6370.diff

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