Added a reference to the Bootstrap 3 form theme
feature #4411 Added a reference to the Bootstrap 3 form theme (javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #4411).

Discussion
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Added a reference to the Bootstrap 3 form theme

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6+
| Fixed tickets | #4296

Commits
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c85a8ac Added a reference to the Bootstrap 3 form theme