[docs] Update links to moved Fleet/Agent files

[dedemorton]

Motivation/summary

Updates links in the APM Server docs in preparation for removing the temporary manual redirect file in elastic/observability-docs#1168

Checklist

N/A

How to test these changes

Navigate to the changed page and click the links to verify that they work.

Related issues

elastic/observability-docs#784

[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-18T19:02:22.706+0000

• Duration: 42 min 17 sec

• Commit: 9149e2d

Test stats 🧪

Test	Results
Failed	0
Passed	6293
Skipped	18
Total	6311

🤖 GitHub comments

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[bmorelli25]

/test

[bmorelli25]

@dedemorton I need to get #6376 merged before merging this. I'll merge in ~30 mins.