Add Java agent section to installation docs

[eyalkoren]

Closes #70

[ghost]

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• Start Time: 2021-12-13T07:22:09.551+0000

• Duration: 7 min 0 sec

• Commit: f8918de

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Test	Results
Failed	0
Passed	78
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Total	78

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[eyalkoren]

run elasticsearch-ci/docs

[AlexanderWert]

run elasticsearch-ci/docs

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/test

[astorm]

+1 approving

[astorm]

@bmorelli25 should we port these changes to the version of this document in the APM Server repo -- or will that not be necessary? (I can't remember the timeline for taking the docs out of the APM Server repo)

[bmorelli25]

@bmorelli25 should we port these changes to the version of this document in the APM Server repo -- or will that not be necessary? (I can't remember the timeline for taking the docs out of the APM Server repo)

I'll have the docs sourced from this repo by the end of the week, so I think we're OK to not copy things over.