workflow: sync tidb-cloud-filesystem from release-8.5

[qiancai]

Summary

• sync the tidb-cloud-filesystem directory from pingcap/docs release-8.5 into markdown-pages/en/tidb-cloud-filesystem/master
• sync TOC-tidb-cloud-filesystem.md into the same English target directory
• keep the Filesystem files out of markdown-pages/en/tidb/release-8.5 during targeted and full updates
• register en, zh, and ja metadata for tidb-cloud-filesystem in docs.json, matching TiDB Cloud Lake; this PR only adds the English sync workflow, with translated content and sync workflows to follow later

Testing

• parsed .github/workflows/update.yml successfully with a YAML loader
• validated docs.json with jq, including equality with Lake language metadata
• ran git diff --check
• ran actionlint; it reported only the existing action-version and tooltip shell warnings also present on upstream/main

Rollout

• the source Filesystem content and TOC are provided by docs: add TiDB Cloud Filesystem product documentation docs#23876 and should be merged before running the English sync workflow
• the zh/ja metadata does not publish translated Filesystem pages until those sources and workflows are added