Adding API Key Link To Line 53, 106, and 258

[nikkiicole]

What does this PR do?

As per customer's request I added a link to API keys in the documentation

Motivation

https://datadog.zendesk.com/agent/tickets/578412

Testing Guidelines

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• [x ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
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