Added third party access optional header instructions.

[loganbertram]

JIRA Ticket:
BB2-3345

What Does This PR Do?

Adds a section describing how data facilitator apps should begin using the optional DATA-END-USER header

What Should Reviewers Watch For?

If you're reviewing this PR, please check for these things in particular:

Check the documentation page to confirm the new section under Access on behalf of a third party and review the content.

Validation

Validate the changes on page at http://localhost:4000/developers/ while running locally

What Security Implications Does This PR Have?

Please indicate if this PR does any of the following:

• Adds any new software dependencies
• Modifies any security controls
• Adds new transmission or storage of data
• Any other changes that could possibly affect security?

• Yes, one or more of the above security implications apply. This PR must not be merged without the ISSO or team security engineer's approval.