add boosted frame notes to usage FAQ (ECP-WarpX#3245)

* add boosted frame notes to usage FAQ
* Update faq.rst
* Update Docs/source/usage/faq.rst
* adjust some wording
* rearrange
* include minimum boosted frame numerics
* be clear that boosted numerics are tricky
* Fix extra newlines

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>
Co-authored-by: Edoardo Zoni <59625522+EZoni@users.noreply.github.com>

Docs/source/usage/faq.rst

@@ -27,6 +27,30 @@ We change the default in ``cmake/dependencies/AMReX.cmake``.
 Note that the tiny profiler adds literally no overhead to the simulation runtime, thus we enable it by default.
 
 
+What design principles should I keep in mind when creating an input file?
+-------------------------------------------------------------------------
+
+Leave a cushion between lasers, particles, and the edge of computational domain.
+The laser antenna and plasma species ``zmin`` can be less than or greater than  the ``geometry.prob_hi``,
+but not exactly equal.
+
+
+What do I need to know about using the boosted frame?
+-----------------------------------------------------
+
+The input deck can be designed in the lab frame and little modification to the physical set-up is needed --
+most of the work is done internally.
+Here are a few practical items to assist in designing boosted frame simulations:
+
+- Ions must be explicitly included
+- Best practice is to separate counter-propagating objects; things moving to the right should start with :math:`z <= 0` and things stationary or moving to the left (moving to the left in the boosted frame) should start with :math:`z > 0`
+- Don't forget the general design principles listed above
+- The boosted frame simulation begins at boosted time :math:`t'=0`
+- Numerics and algorithms need to be adjusted, as there are numerical instabilities that arise in the boosted frame. For example, setting ``particles.use_fdtd_nci_corr=1`` for an FDTD simulation or setting ``psatd.use_default_v_galilean=1`` for a PSATD simulation. Be careful as this is overly simplistic and these options will not work in all cases.  Please see the :ref:`input parameters documentation <running-cpp-parameters>` and the :ref:`examples <usage-examples>` for more information
+
+An in-depth discussion of the boosted frame is provided in the :ref:`moving window and optimal Lorentz boosted frame <theory-boostedframe>` section.
+
+
 What kinds of RZ output do you support?
 ---------------------------------------