Update git submodule "docs/en/_theme"

This applies the following fixes to DBAL web docs:

- Fix broken ServerGrove image on footer
- Fix occasional link overlap
- Delete .DS_Store file in bootstrap folder
- Finish PR #3 "<ol/> styles fixed" (margin-bottom)
- Fix <ol/> styles
- Synchronize docs layout menu with main site.
- Show versions only when having more than one.

Fixed oracle bug that primary key is missing if index_name != constraint_name

SQLParserUtils::getPlaceholderPositions fails if there are quoted strings containing only backslashes

Add immutable date types

[DBAL-990] Try platform detection without database name as fallback

Closes #1882

FIX Sequence::isAutoIncrementsFor now case insensitive

Merge MySQL 5.7.9 (GA) semantics into MySQL57Platform

Adding PDO as hard dependency as per discussion in #808

Close #713
DBAL-1030

Added missing filtering of table name assets in DB2SchemaManager

Close #2704

#2704 - Removed hacky handling of error handler restore operations - restoring twice in the test instead

MySQL: Fix primary key alteration when adding new columns

Fix docs for Doctrine\DBAL\Id\TableGenerator