Build developer docs in strict mode

[florimondmanca]

What does this PR do?

Enable --strict mode when building docs.

Motivation

In this mode, mkdocs build errors out if any warnings were discovered while building:

Determines how warnings are handled. Set to true to halt processing when a warning is raised. Set to false to print a warning and continue processing.

https://www.mkdocs.org/user-guide/configuration/#strict

Prevents issues like #8150 from happening again. (We don't typically watch build logs to see if there are warnings, so they could sneak in unknowingly.)

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 #8152 (4d1c45a) into master (414b2f8) will decrease coverage by 2.88%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
fluentd/tests/mock/fluentd_version.py
go_expvar/datadog_checks/go_expvar/__about__.py
elastic/datadog_checks/elastic/utils.py
openstack_controller/tests/test_metrics.py
etcd/datadog_checks/etcd/__init__.py
http_check/datadog_checks/http_check/utils.py
druid/tests/conftest.py
gearmand/tests/conftest.py
mapreduce/datadog_checks/mapreduce/mapreduce.py
clickhouse/datadog_checks/clickhouse/__about__.py
... and 848 more