Add metadata_entrypoint to API reference

[florimondmanca]

What does this PR do?

Document @AgentCheck.metadata_entrypoint on docs site.

Motivation

Prompted by #6916 (comment), refs #6084

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #6938 into master will increase coverage by 2.99%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
disk/tests/common.py	64.28% <0.00%> (-35.72%)	⬇️
disk/tests/conftest.py	85.71% <0.00%> (-11.43%)	⬇️
disk/datadog_checks/disk/disk.py	83.66% <0.00%> (-3.59%)	⬇️
disk/tests/test_filter.py	96.45% <0.00%> (-3.55%)	⬇️
dns_check/datadog_checks/dns_check/dns_check.py	86.45% <0.00%> (-2.09%)	⬇️
...tadog_checks/rethinkdb/document_db/transformers.py
fluentd/datadog_checks/fluentd/fluentd.py
activemq_xml/tests/test_activemq_xml.py
envoy/tests/test_parser.py
system_swap/tests/conftest.py
... and 547 more