docs: add lance-flink connector documentation site

[fightBoxing]

Summary

Adds docs/src/ to the lance-flink repository, following the lance-spark docs template, so the connector can be aggregated into the lance.org integrations site (see lance-format/lance#8987).

Structure

docs/src/
├── .pages                    # Welcome → Install → Config → Performance → Operations
├── index.md                  # Introduction / Features / Quick Start
├── install.md                # 1.18/1.19/1.20, deps, JNI platforms, build
├── config.md                 # full ConfigOption reference (read/write/index/vector/S3/hadoop.*/namespace)
├── performance.md            # index selection, search tuning, write amplification
└── operations/
    ├── ddl/    create-catalog.md + create-table.md
    ├── dql/    select.md + vector-search.md + time-travel.md
    └── dml/    insert-into.md

Notes

• Unimplemented capabilities are explicitly noted as unsupported / in progress (ALTER TABLE, CREATE INDEX, UPDATE, DELETE, PK/upsert).
• DELETE / CDC changelog correctness is tracked in [epic] DELETE via PRIMARY KEY (reworked after adversarial review, replaces PR #19) #63 and [design risk] DELETE is silently dropped when +I(k) and -D(k) arrive in the same checkpoint #74.

Related: lance-format/lance#8987