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
Added docs about groupsequenceproviders
[fixed] Fixed example code so that custom authentication providers only clear their own tokens
Fix doubled words
New ANSICON link
Merge pull request #2836 from jbafford/typo-fixes

Fix doubled words
Merge pull request #2839 from dosten/ansicon_link

New ANSICON link
fix security context example code
add YAML and XML example for Required and Optional field constraints in a Collection constraint
complete constraint XML examples
fix label
fix cookbook article link since the virtual option doesn't no longer exist
fixing link to the translation domain section in cookbook article