Make plugins webpage separate database, dataset, and global extensions

I think this makes it much easier to digest the list of extensions.
I'd like to do a similar organization for model plugins, but
they unfortunately do not have a clean breakdown like this.
Add guide for ordering code with Sequel

This should hopefully give guidence and prevent people from
reporting that their model classes don't work when they are
loaded before the database connection has been made.
Deprecate global use of to_dot extension

This should be a dataset extension, since it adds a method to
Sequel::Dataset.
Deprecate global use of null_dataset extension

This adds methods to Dataset, and similar to pagination it should
be a dataset extension.
Mention that Dataset#set was added to sequel_3_dataset_methods
Deprecate Model.add_graph_aliases, .insert_multiple, .query, .set_overrides, .set_defaults, .to_csv, and .paginate

These methods either don't make sense (add_graph_aliases) or
are part of extensions and not the core dataset code.

Add some notes to remove methods from QUERY_METHODS and
ACTION_METHODS in 4.0, so Model::DATASET_METHODS won't need to
be updated.
Add specs for model class dataset methods

All of these methods are created via metaprogramming, so the
coverage tests can't detect which ones had coverage.