doc: remove unnecessary files

Most of them are already moved to the site repo: https://git.picodata.io/picodata/picodata/docs/-/tree/97c3e35ec/docs

• clustering.md -> https://docs.picodata.io/picodata/clustering/
• glossary.md -> https://docs.picodata.io/picodata/glossary/
• cli.md -> https://docs.picodata.io/picodata/cli/
• discovery.md -> https://docs.picodata.io/picodata/discovery/
• sharding.md -> /dev/null
• topology.md -> https://docs.picodata.io/picodata/deploy/

Related: docs!141 (merged)