Duplicate siunitx.dtx as siunitx-v2.dtx

This is the first step in the v3 rewrite. The plan here is to recode
the mechanisms a little at a time into various new files. Thus from
here 'onward' the main branch will most likely not be useable for some
time (until the various pieces of work are done and everything is
ready to be reassembled).
Remove 'content' from siunitx.dtx

This is now set up as a 'template' `.dtx` file to use to construct
the various new files that will be created. The file siunitx-v2.dtx will
be used over time to provide/track the original code for the conversion
process.
Use \pkg rather than \textsf (logical mark up)

How did this not get picked up earlier in any of the LaTeX3 files?
Tidy up the .gitignore file

Various items had collected up here: start from scratch using only
file types that actually apply to this package.
New file siunitx-units.dtx (unit parsing/formatting submodule)

This will be the core of the main competence of `siunitx`: formatting
units. The aim is that this submodule could at some stage be spun out
as a stand-alone `units` module with only minor alterations (probably
not until there is a stand-alone LaTeX3 kernel to make this
sensible/desirable).
Typesetting siunitx.dtx should now state that this is code-level info
Minor change to code formatting in titles

This matches the style typically used in 'normal' LaTeX documents.
Add siunitx.tex file to contain user documentation

With a number of `.dtx` files planned it makes more sense to have the
user documentation in an entirely separate file to the code docs. This
should keep all of the files manageable.
Meta-comment in LaTeX file does not need comment chars
Add *.sty to .gitingore
Modify meta-comments so they are not copied by DocStrip

While for the LaTeX3 Project sources it is standard to copy all of
this info, my personal preference is not to do so.
Add an initial .ins file

The exact DocStrip structure will evolve over time: for example, its
not clear whether to have 'early' and 'late' code in the main `.dtx`,
whether to extract into a single `.sty`, _etc._
Remove .ins files from "make clean"

While the build scripts will need quite a bit of work later on, at the
moment it is important that `make clean` continues to work correctly.
Remove 'early' loading of expl3/xparse

The driver is using `l3doc` so this is redundant anyway (`xparse` should
be loaded 'normally' if/when needed). Loading `expl3` can happily be
covered in the 'proper' code sections as everything is built up, and
is only currently appropriate in the main file anyway (assuming DocStrip
is used to make a single `.sty`).
Add a line break in title for main .dtx file
Add section for code typesetting
Main DocStrip guards and package identification

This is all simple enough that there is no point removing it from
the v2 file.
Actually load expl3
A few comments on the basic DocStrip set up
Basic DocStrip set up for siunitx-units