[8.0] docs: link to new APM book (#1850)#1865

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bmorelli25 merged 1 commit intoelastic:8.0from bmorelli25:backport/8.0/pr-1850

Oct 29, 2021

+49-57

Commits on Oct 29, 2021

docs: link to new APM book (elastic#1850 )
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