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
Add missing class
Merge pull request #2847 from piotrantosik/patch-2

Add missing class
Added docs about immutable dispatcher
Documented htmlPattern option
Unified autoloading
gc_maxlifetime default value

The correct value of parameter gc_maxlifetime is not 14400 but 1440.
Merge pull request #2880 from petajaros/patch-1

gc_maxlifetime default value
Merge pull request #2863 from WouterJ/issue_2858

Added docs about immutable dispatcher
[#2863] Fixing some ordering of documents
Merge pull request #2864 from WouterJ/issue_2833

Documented htmlPattern option
[#2864] Small tweaks thanks to @xabbuh on new Regex:htmlPattern option docs
Merge pull request #2867 from WouterJ/issue_2331

Unified autoloading in Sf2 <> Sf1 article
[#2867] Minor tweaks