Initial Documentation commit (warning: HUGE!)

As the title states, thus begins "official" work on PhotoDemon
documentation.  Primary purpose of this commit is to add a new Docs
folder to the repo, with full folder and .md file layout for an initial
set of documentation.

(Content itself is missing, but will obviously be added over time.)

Lots of notes for future reference:

- For a documentation framework/CMS, I've decided to go with the lovely
Daux.io:

https://github.com/justinwalsh/daux.io
http://daux.io/

- A live version that (roughly) mimicks the repo state is now up at
photodemon.org/help.  I'm running a live Daux instance for testing
purposes, but once I've gotten the documentation fleshed out, I'm going
to switch to a static build.  Besides reducing server load, this will
ensure that the online version stays in sync with the latest official
release.

- Daux uses the layout and contents of the /Docs folder for full site
generation.  So if testers, authors, translators, etc ever want to run
their own local copy of Daux (as I do for editing), they can simply drop
this repo's /docs folder into their localhost folder, and assuming
they've also unzipped a copy of Daux there, they can instantly view the
documentation in a roughly identical format to photodemon.org's copy.  I
love this.

- Localization is being implemented from the beginning.  I have no idea
if people will ever be crazy enough to try and translate all these
files, but at least I'm planning for it.

- Help integration with PD itself will be handled in 6.4.  My goal for
6.4's release is to not only have contextual help implemented in PD, but
also have documentation fleshed out for all major program functions.
Because of this, I won't be talking about help (or advertising
photodemon.org/help) until after 6.2's release.

- For organizational reasons, all folders and md files are prefixed by
numbers.  Daux strips these when generating permalinks and HTML, but it
uses the numbers to know how folders and files fit into the hierarchy.
I have arbitrarily used multiples of 5 as prefixes, which gives me some
wiggle room for inserting new pages in the future without breaking
existing permalinks.

- After some deliberation, I have decided *not* to store documentation
images in the repo.  I'm looking at potentially thousands of images,
which would bloat the repo greatly, so I'll handle images separately.

(FYI, all image links in the canonical English documentation will
reference a root images folder.  I am operating under the assumption
that images will *not* generally need to be localized, so this allows
other languages to copy all existing image URLs and still have them
work.)

- I'm sure there are other useful things I'm forgetting to say, but as
there will be *many* more documentation commits in the future, this will
do for now.  :)