update slack migration docs

[vish9812]

• Updated Slack export terminology from "Regular export"/"Corporate export" to current Slack terms ("Public channels export"/"All channels and conversations export") and corrected the UI navigation path
• Removed the slack-advanced-exporter section and all references — Slack exports already include emails and file download links, making this tool unnecessary
• Removed the Additional Tools subsection (mm-emoji, mmimportjs, slack-migrate-pinned-posts) as they are archived, broken, or no longer maintained
• Added mmetl check slack validation step before running the transform
• Added documentation for useful mmetl transform flags: --skip-attachments, --allow-download, --default-email-domain, --skip-empty-emails, --discard-invalid-props
• Added mmctl import validate step before processing the import
• Fixed incorrect command mmctl import list process to mmctl import job list
• Fixed typos and minor errors throughout the document

PS: I've not tested it with the Enterprise export. So, no changes around that.

[Copilot]

Pull request overview

This PR updates the Slack-to-Mattermost migration documentation to improve accuracy, clarity, and alignment with current tooling. The updates remove references to the deprecated slack_advanced_exporter tool, add a validation step using mmetl check slack, correct terminology around Slack export types, fix numerous spelling and grammar errors, and refine configuration guidance.

Changes:

• Removed deprecated slack_advanced_exporter tool instructions and streamlined to focus on the officially supported mmetl tool
• Added validation step for Slack exports before transformation using mmetl check slack
• Corrected multiple spelling and grammar errors throughout the document (necessary, enough, additional, associated, Starred, Threaded conversations)
• Updated Slack export type terminology to "Public channels export" and "All channels and conversations export" with clarified descriptions
• Fixed configuration guidance for TeamSettings.MaxChannelsPerTeam and removed extra space in ElasticsearchSettings.EnableIndexing

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[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 92255d6

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 92255d6

[wiggin77]

Are these tools no longer needed? If so, can we add a line explaining why?

[vish9812]

It's mentioned in the PR description: 🙂

• Removed the Additional Tools subsection (mm-emoji, mmimportjs, slack-migrate-pinned-posts) as they are archived, broken, or no longer maintained

[wiggin77]

I mean explaining to the reader why these tools they've used in the past are no longer needed.

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA c88dbd1

[esethna]

@wiggin77 can you help with a re-review on this as needed?

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 4d21d1a

[wiggin77]

LGTM 👍 @fmartingr should also have a look before merging.