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
Refactored OptionsResolver docs to have a better example
Added cookbook page about sessions directory.
Add some clarification to Quick Tour Working with Environments

Also, for the session token: identify what the "weird" hex number is, and and replace "weird" with "long" (a more apt description).
removed duplicate timestamp limitation notice
added timestamp limitation notice for datetime
Merge pull request #2674 from jbafford/i2672

Add some clarification to Quick Tour Working with Environments
Added namespace declaration to MainController.php.
Changed </components/yaml/introduction> to </components/yaml/format> per PR feedback.
Fixed wrong used class
Rewrite validation groups explanation.
[Cookbook][Session] Fixed headline in proxy_examples
Added article on how to simulate saving locale in session
Added docs for enum nodes