docs: rename platform index.md to readme.md (#25215)

lib/modules/platform/azure/readme.md

lib/modules/platform/bitbucket-server/readme.md

lib/modules/platform/bitbucket/readme.md

lib/modules/platform/codecommit/readme.md

lib/modules/platform/gitea/readme.md

lib/modules/platform/gitlab/readme.md

lib/modules/platform/local/readme.md

tools/docs/platforms.ts