Document channel member management flows

[svelle]

Summary

• rework the web/desktop "Manage channel members" article to show the current ways to add people to a channel
• add fresh full-window screenshots for the sidebar menu, channel header menu, members pane, View Info sidebar, @mention prompt, remove-member flows, and profile popover
• update the desktop profile-popover subsection so it matches the currently available UI shown in the screenshot
• expand the mention and remove-member sections with current desktop screenshots and matching instructions

Testing

• launched a local Mattermost instance against PostgreSQL from a tarball install
• seeded and uploaded visible profile images for test users so screenshots render clearly
• captured all new desktop screenshots from the live UI and updated the article around those concrete flows

Notes

• The profile popover in this environment shows a direct-message action rather than an explicit "Add to a channel" action, so the article text was adjusted to stay accurate to the current UI.

  

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Newest code from mattermost has been published to preview environment for Git SHA ffddc39

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[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 1ef7d66

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA f968808

[esethna]

Thanks for the PR @svelle. A few things:

1. The images used are very zoomed out and uncropped, it's hard to identify what the user should be looking at in the image.
2. The number of consecutive images makes the doc quite hard to read. Wondering if GIFs could be a good way to walk through these methods while keeping the page readable and scanable.

For example, as an idea, the many way to add users to channels could be numbered list followed by a single GIF showing them all.

Alternatively, do we need an image for each method or can we slim it down to kee the page readable?