Document undocumented dolt SQL procedures

Doltgres re-uses dolt's `DoltProcedures` slice — see [`server/functions/dolt_procedures.go`](https://github.com/dolthub/doltgresql/blob/main/server/functions/dolt_procedures.go) — so it ships every procedure dolt has as a callable function. But the doltgres docs at [`reference/sql/version-control/dolt-sql-functions.md`](https://github.com/dolthub/docs-2/blob/main/site/doltgres/src/content/reference/sql/version-control/dolt-sql-functions.md) only cover 22 of the ~37 available.

Missing from doltgres docs:

- `dolt_commit_hash_out` (was documented for dolt in #58; doltgres equivalent still pending)
- `dolt_count_commits`
- `dolt_rm`
- `dolt_stash`
- `dolt_stats_flush`, `dolt_stats_gc`, `dolt_stats_info`, `dolt_stats_once`, `dolt_stats_purge`, `dolt_stats_restart`, `dolt_stats_stop`, `dolt_stats_wait`, `dolt_stats_timers`
- `dolt_thread_dump`
- `dolt_update_column_tag`

Most of these can be ported with light adaptation from the dolt-sql-procedures page since the implementation is the same; the doltgres calling convention is `SELECT … FROM` instead of `CALL`. Add the resulting anchors to `site/doltgres/src/content/reference/sql/version-control/sql-extensions.md` once each is documented.

Surfaced during the source-vs-docs cross-check for #43 (see #64).