docs: document float SST encoding at table creation

[discord9]

What changed

Document experimental_sst_float_field_encoding in the CREATE TABLE reference: accepted values, default behavior, FLOAT/DOUBLE field scope, an opt-in creation example, and workload-dependent performance. Clarify that ALTER is not supported. This documents the existing implementation merged in GreptimeTeam/greptimedb#9069, not the unmerged ALTER proposal #9077.

Scope

• Documentation versions: Nightly
• Languages: English and Chinese

Verification

• git diff --check: passed.
• Checked option parsing and Parquet writer behavior against GreptimeDB source.
• Local build could not run because Node and dependencies are unavailable; build validation is left to CI.
• No links, anchors, or navigation changed.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed.

Links, anchors, and navigation checklist items are not applicable: none changed.

[killme2008]

If this new encoding is useful for metrics, do we need to add it to the user guide doc?

https://docs.greptime.com/user-guide/ingest-data/for-observability/prometheus/#performance-tuning

[killme2008]

LGTM almostly