Document undocumented dolt SQL procedures

[tbantle22]

Doltgres re-uses dolt's DoltProcedures slice — see 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 only cover 22 of the ~37 available.

Missing from doltgres docs:

• dolt_commit_hash_out (was documented for dolt in Document dolt_commit_hash_out() stored procedure #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).