Fixed #22796 -- Added more basic explanations of forms

Now explains some more basic form handling before discussing Django;
simplifies initial examples; defers explanation of more advanced
aspects until later in the document
The important distinction is that GET should not have side effects (e.g. a search would return the same results each time), while POST should be used for something does have side effects (e.g. submitting a form that changes some state in the database).