doc: Add info about plugin uninstallation

[lantoli]

Description

Add info about plugin uninstallation. Also fix the license link.

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[Copilot]

Pull Request Overview

Enhances the documentation by adding plugin uninstallation instructions and clarifies the local development setup process.

• Adds plugin uninstallation command to the main README
• Provides detailed local development workflow in CONTRIBUTING.md
• Fixes license file reference from LICENSE.md to LICENSE.txt

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
README.md	Adds uninstallation section and corrects license file reference
CONTRIBUTING.md	Enhances local development setup with uninstallation prerequisites and detailed workflow steps

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