chore: add env variable documentation for backup retention limit (#2976)

## Description 

Provide a concise summary of the changes made in this pull request
- Added documentation for backup retention: new
APPSMITH_BACKUP_ARCHIVE_LIMIT env var describing count-based cleanup,
default limit of 4, timestamped backups, and pruning of oldest files
after each run.

## Pull request type

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- [ ] Review Fixes
- [ ] Documentation Overhaul
- [ ] Feature/Story
    - Link one or more Engineering Tickets
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- [ ] A-Force
- [ ] Error in documentation
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## Documentation tickets

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## Checklist

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- [ ] Checked for Grammarly suggestions.
- [ ] Adhered to the writing checklist.
- [ ] Adhered to the media checklist.
- [ ] Verified and updated cross-references or added redirect rules.
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Author: sebastianiv21

website/docs/getting-started/setup/environment-variables.md

@@ -513,6 +513,14 @@ Specify a `5-value` cron expression to define the schedule for automatic backups
 
 </dd>
 
+##### `APPSMITH_BACKUP_ARCHIVE_LIMIT`
+
+<dd>
+
+Controls how many recent backup archives are retained. Backups use a count-based retention system (not time-based): each run creates a new timestamped file in `/appsmith-stacks/data/backup/`, and after every successful backup the cleanup process prunes the oldest files if the total exceeds this limit. The default is `4`, ensuring storage does not grow unbounded while keeping the most recent backup history. Backups are never overwritten; older files are simply deleted once the limit is surpassed.
+
+</dd>
+
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