Add `Delegation Removal` subsection to Repository Operations in specification Documentation

[shubhusion]

Description: This PR adds a new subsection to the repository operations documentation titled "Delegation removal." The new subsection outlines the steps required to remove a delegatee, including the deletion of targets, the delegatee's targets metadata, and updating the delegator's targets metadata. Additionally, it specifies retaining snapshot metadata until a reset is needed and provides references to relevant sections for more details.

Changes:

• Added a new subsection ### Delegation removal {#deleting-delegation} to the repository operations documentation.
• Included step-by-step instructions for delegation removal.
• Provided references to Section 5.3.11 for more details on updating root metadata and handling fast-forward attacks.
• Provided references to Section 6.3.2 for more details on updating snapshot metadata.

Reviewers: @trishankatdatadog @JustinCappos

Issue: Fixes #262

Thank you!