Browse channels page needs animated GIFs for mobile app workflows

[Rutam21]

Context

To help visual learners learn how to perform tasks using the Mattermost mobile app, add visuals and short animated GIFs on the Browse channels page.

Parent Issue: #6657

Docs Request

• Add appropriate visuals/GIFs for all the steps in the Browse Channels section under the mobile app tab.

Additional Notes

• Use the latest Mattermost mobile app and the standard Mattermost Denim theme to capture all workflows.
• Avoid drawing lines, arrows, boxes, or callouts on images.
• Don't forget to include ALT Text descriptions for all new image assets.
• Use existing docs pages with images as a starting point for code guidance. The Docs team can help provide RST syntax support for publication.
• Save static image files in PNG format within the docs repository's /images directory.

QA Validation Criteria

In a generated docs preview:

• Visiting the docs page results in appropriate new mobile-specific screenshots in each subsection from the Mobile tab.

[Rutam21]

@cwarnermm Please assign this issue to me. Thanks.