docs(multi-executor): remove reference to single executor (#295)

[Dimfacion]

Proposed changes

• Removing reference to only single connector

Related issues

• Closes docs(multi-exec): explain the possibility to run multiple instances of the same item from the catalog #295

How to test this PR

Checklist

• The PR title follows the Conventional Commits convention type(scope?): description (#issue)
• I signed my commits
• This PR is linked to an issue
• I consider the submitted work as finished
• I tested the code for its functionality
• I added/updated the relevant documentation
• Where necessary, I refactored code to improve the overall quality

[Copilot]

Pull request overview

Updates the Integration Manager documentation to remove wording that implied only a single managed instance/executor could exist, aligning the docs with the “multiple instances from the catalog” capability referenced in #295.

Changes:

• Adjusted the “Browsing the catalog” section to point readers to the dedicated pages for managing already-deployed items.
• Relaxed the migration prerequisites by removing the “no existing managed instance” constraint for external connectors.

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