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.

_theme @ 6f1bc8be

-Subproject commit 5aa024c9284228ed76c7a9aa785229cf19f396e3
+Subproject commit 6f1bc8bead17b8032389659c0b071d00f2c58328