docs: add realtime engine (RTE) getting-started guide (#4548)

Adds a user-facing realtime engine (RTE) page at
`docs/web/tables/realtime-engine.mdx`, listed under **Management** after
Branches. It is adapted from the internal WAL field guide
(lancedb/sophon#6213), with the deployment and operations material left
out.

The page covers:
- what RTE is and how it works (memtable → SSTables → compaction; reads
merge the base table with the fresh tier)
- getting started: `set_unenforced_primary_key`, indexes, `enable_rte` /
`ALTER TABLE ... SET RTE`, `merge_insert` upserts and queries, partial
updates, deletes, and `checkpoint_rte` / `disable_rte`
- performance: write throughput and shard scaling, and read latency with
and without RTE
- a note on operations not yet supported (time travel, branching,
`count_rows`, row IDs, `TABLESAMPLE`)

Also aligns related docs with current RTE naming and behavior:
- `sql/ddl/table.mdx`: `SET MEMWAL` / `UNSET MEMWAL` → `SET RTE` /
`UNSET RTE`
- `maintenance/background-jobs.mdx`: job names are `memwal_compaction`
and `memwal_catchup_<type>`
- `feature-engineering/materialized-views.mdx`: MemWAL → realtime engine
- Python `AsyncTable.enable_rte` /
`RTEWriteSpec.with_maintained_indexes` docstrings: by default, indexes
created after enabling RTE are maintained too

The examples are written directly in the page rather than generated from
`docs/web-tests`, because RTE needs an Enterprise server to run.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Co-authored-by: XY Zhan <zhaner08@hotmail.com>

python/python/lancedb/_lancedb.pyi

@@ -850,9 +850,9 @@ class RTEWriteSpec:
     @staticmethod
     def unsharded() -> "RTEWriteSpec": ...
     def with_maintained_indexes(self, indexes: Optional[List[str]]) -> "RTEWriteSpec":
-        """Set which indexes the RTE keeps up to date. None resolves every
-        index on the table at install, failing if one cannot be maintained;
-        a list is verbatim, empty means none."""
+        """Set which indexes the RTE keeps up to date. None is every index on
+        the table, now and later, skipping any it cannot maintain; a list is
+        verbatim, empty means none."""
         ...
     def with_writer_config_defaults(self, defaults: Dict[str, str]) -> "RTEWriteSpec":
         """Return a copy of this spec recording the given default

python/python/lancedb/table.py

@@ -5564,12 +5564,12 @@ class AsyncTable:
         via [`set_unenforced_primary_key`]; bucket sharding additionally
         requires it to be the single column being bucketed.
 
-        By default the RTE maintains every index on the table, resolved
-        here — a snapshot, so an index created afterwards needs the spec unset
-        and set again. This fails if one cannot be maintained; name the set
-        with ``with_maintained_indexes`` to install anyway. That pins an exact
-        set (a still-building index is rejected, not omitted); ``[]`` maintains
-        none.
+        By default the RTE maintains every index on the table, re-read as the
+        table changes, so an index created later is maintained too; one of a
+        kind the RTE cannot mirror is skipped. Name the set with
+        ``with_maintained_indexes`` to pin it instead: each name must already
+        exist and be maintainable (a still-building index is rejected, not
+        omitted); ``[]`` maintains none.
 
         Parameters
         ----------