feat(docs): Enhance Bigtable GoogleSQL DML clarity
**Description:**
This PR updates the `bigtable-sql` tool documentation to provide a
clearer understanding of GoogleSQL's DML capabilities within Bigtable.
**Changes Made:**
- Added a note indicating that Bigtable's GoogleSQL dialect only
supports `SELECT` DML operations.
- Explicitly states that `INSERT`, `UPDATE`, and `DELETE` DML statements
are not supported.
- Updated the external link to directly reference the "Use cases"
section in the official Google Cloud Bigtable GoogleSQL overview, where
this limitation is detailed.
**Reasoning:**
The previous documentation, while linking to the overview, did not
explicitly highlight the DML limitations. This clarification will
prevent potential confusion for users expecting full DML support,
ensuring the documentation accurately reflects Bigtable's capabilities
with GoogleSQL.
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Co-authored-by: Twisha Bansal <58483338+twishabansal@users.noreply.github.com>
