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
[Process] Doc for output flush methods.
[Process] Rewriting to avoid wrong sentences.
fixed XML examples
Updated rounding_mode documentation to reflect changed ROUND_ constants.
fixing commands in the reverse engineering example
Clarified how to load translations from db

Added link the the translation loader tag documentation.
Removed trailing whitespace.
Change 'translation loader tag' to 'translation.loader tag'
Switched regular link to doc reference
[Process] Update docs for modifications.
Document possibility to use ContainerAwareTrait

In addition to https://github.com/symfony/symfony/pull/8488
Small typo

Fixed small typo (extra "`")