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
change non-existent markup type php+html to html+php
change forward example to show how to create a sub-request explicitly without using the deprecated format() method of the HttpKernel class
Added a couple of missing semicolons
Fix typo

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2+
| Fixed tickets |
Added tip
Fix property path default value

The default value of a property path is its name, not its value.
Update dynamic_form_modification.rst
swap use statement to sort alphabetically
Merge pull request #2911 from xabbuh/fix-markup-name

change non-existent markup type php+html to html+php
Merge pull request #2912 from xabbuh/issue-2910

change forward example to show how to create a sub-request explicitly wi...
Merge pull request #2916 from rickpastoor/patch-1

Added a couple of missing semicolons
Merge pull request #2919 from lucascourot/patch-3

Fix typo
Update link hashes to propel site.