Hardcopy of pybind11 instead of using git submodules (#552)

* removed pybind as submodule
* added hardcopy of pybind11 2.10.0
* rename pybind11 folder to avoid conflicts when changing branch

Co-authored-by: Dhanya Thattil <dhanya.thattil@psi.ch>

libs/pybind/docs/index.rst

@@ -0,0 +1,48 @@
+.. only:: latex
+
+   Intro
+   =====
+
+.. include:: readme.rst
+
+.. only:: not latex
+
+    Contents:
+
+.. toctree::
+   :maxdepth: 1
+
+   changelog
+   upgrade
+
+.. toctree::
+   :caption: The Basics
+   :maxdepth: 2
+
+   installing
+   basics
+   classes
+   compiling
+
+.. toctree::
+   :caption: Advanced Topics
+   :maxdepth: 2
+
+   advanced/functions
+   advanced/classes
+   advanced/exceptions
+   advanced/smart_ptrs
+   advanced/cast/index
+   advanced/pycpp/index
+   advanced/embedding
+   advanced/misc
+
+.. toctree::
+   :caption: Extra Information
+   :maxdepth: 1
+
+   faq
+   benchmark
+   limitations
+   reference
+   cmake/index