PMM-14062: Drop five experimental dashboards; update AGENTS and MkDocs

[mattiasimonato]

https://perconadev.atlassian.net/browse/PMM-14062

• Links to relevant pull requests

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4342-78e697c
Client docker: perconalab/pmm-client-fb:PR-4342-78e697c
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4342-78e697c
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4342-78e697c.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4342-78e697c&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4342-78e697c.tar.gz

[JNKPercona]

API tests have succeded: https://pmm.cd.percona.com/job/pmm3-api-tests/6033/

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4342-a086b30
Client docker: perconalab/pmm-client-fb:PR-4342-a086b30
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4342-a086b30
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4342-a086b30.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4342-a086b30&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4342-a086b30.tar.gz

[JNKPercona]

API tests have succeded: https://pmm.cd.percona.com/job/pmm3-api-tests/6036/