Add section on removing session keys

[eshaben]

Description

This PR adds a section to the collator account management page on how to remove session keys for collators who no longer want to be a collator

Checklist

• If this requires translations for the moonbeam-docs-cn repo, I have created a ticket for the translations in Jira
• If images have been added, I have run the compress-images.py script to compress the images.

After Translation Requirements

• No additional PRs are required after the translations are done

[albertov19]

Maybe we should recommend collators to officially stop collating and leave the collator set before they remove the keys, if not, I'm not sure what would happen and if they are still occupying a collator slot?