Add documentation on Database introspection methods to internals.rst

[simonw]

internals.rst will be landing as part of #683

[simonw]

Here's what I have so far:

db.name

db.size

db.mtime_ns

await db.table_exists(table)

await db.table_names()

await db.table_columns(table)

await db.primary_keys(table)

Returns the names of the columns that are part of the primary key for this table.

await db.fts_table(table)

Returns the name of the FTS table associated with this table, if one exists.

await db.label_column_for_table(table)

Returns the label column that is associated with this table - either automatically detected or using the ``"label_column"`` key from :ref:`metadata`.

await db.foreign_keys_for_table(table)

await db.hidden_table_names()

await db.view_names()

await db.get_all_foreign_keys()

await db.get_outbound_foreign_keys(table)

await db.get_table_definition(table)

await db.get_view_definition(view)

[simonw]

This page here: https://datasette.readthedocs.io/en/latest/internals.html

[simonw]

More important is to document execute() - see #685

[simonw]

This documentation is live here: https://datasette.readthedocs.io/en/latest/internals.html#database-introspection