ODBC Compatibility: Add doc for custom header for table type

**ETA:** _Provide an estimated completion date for the feature (if available)_
* Not urgent

**PR:** _Provide PR URL(s) for this feature (if available)_
* https://github.com/influxdata/influxdb_iox/pull/15132

# What

Add document for the FlightSQL command `GetTables` new header `x-influxdata-base-table-type`. Valid values are `BASE TABLE` (default, for JDBC compatibility) or `TABLE` (ODBC compatibility).

# Why

https://github.com/influxdata/influxdb_iox/pull/15132 added support for configuring the base table type returned by FlightSQL `GetTables` requests through a new HTTP header `x-influxdata-base-table-type`. This allows clients to specify whether they want base tables to be reported as `BASE TABLE` (default) or `TABLE`.

This is mainly to support generic ODBC connector on Power BI and Excel on Windows, where base table type is `TABLE` instead of `BASE TABLE`.

## Usage

Clients can now send FlightSQL requests with the header:
```
x-influxdata-base-table-type: TABLE
```
or
```
x-influxdata-base-table-type: BASE TABLE
```

* If no header is provided, the system defaults to "BASE TABLE" for backward compatibility.
* If an invalid value is specified, the client will receive an error like `Invalid argument: Invalid base table type: {invalid_value}. Valid values are 'BASE TABLE' or 'TABLE'`

##### Relevant URLs
- _Provide relevant URLs (issues, community threads, existing doc pages, etc.)_
- Original issue: https://github.com/influxdata/influxdb_iox/issues/15118
- Note that the `SHOW TABLES` query is not affected by the new header; the table types are always `BASE TABLE`. See more details in https://github.com/influxdata/influxdb_iox/issues/15199

Start with the FlightSQL GetTables documentation entry point and review the linked PR for the implemented header behavior. Document the x-influxdata-base-table-type values, default, invalid-value error, and the distinction from SHOW TABLES; the work is complete when clients can find and use this guidance.

Written by the indexing model from the issue text.