Update book/forms.rst

Fix typo (missing ";") in source code sample for dynamic validation groups
Updated .gitignore to ignore Sphinx files
Bootstrapped Form component chapter
Introduction chapter for the Form component
[WIP][#1883] Adding more and more details for the form introduction

Still a WIP because:

a) Bernhard will certainly need to review it
b) I would like to re-read it - I felt that there are still gaps - too many details here, too few details there
c) This is part of a larger, on-going project to refactor the forms docs
d) We need to bootstrap the validation and translation component docs, and other things need to be refactored to make sense
[Form] Making updates per @bschussek and fixing some markup issues
Removing .gitignore file - putting the sphinx build files right in this directory is just one option
[Form] Removing constants, just to avoid confusion about them being somehow important internally
fix typo - missing word
typo - change  to
change headers example to use a request header (not a response header)
decorate php function with link to manual
add more descriptive text for book link
Shorten line under 80 chars
lower case request since it's not talking about the class
change to URI to be more consistent. fix typo. lower case Session
change query to request to be more accurate
lower case request
change final bullet to end with period
backtick Request. link to Request API