You should probably start with Leslie Lamport's classic. It's clear, thorough, and concise. It also has a reference section in the back where you can look things up quickly if you forget some particular syntax. Plus the drawings in the book are really cute! After reading this book you can move on to some of the more modern tutorials people have linked to.
Googling and glancing at it this seems decent:
https://en.wikibooks.org/wiki/LaTeX
(I mostly learned LaTeX from Lamport's 1994 LaTex book in the mid 2000s, but wouldn't recommend it today.)
I recommend buying Lamport and reading it. Then write a paper using it. If you need stuff not in Lamport, then the LaTeX Companion is the go-to source.
Not what you wanted, but for reference these books are infinitely better than all the videos in the world plus stackoverflow.
I don't know whether there is a package for using Georgia with LaTeX—it looks like there can be, but not with math support. [https://tex.stackexchange.com/questions/401990/how-do-i-use-the-georgia-font-in-pdftex\]
There are many tutorials for using LaTeX online (as well as books like LaTeX: A Document Preparation System https://www.amazon.com/LaTeX-Document-Preparation-System-2nd/dp/0201529831.
I recommend to students that they start with the online Overleaf installation of LaTeX, as that has most packages already installed and avoids debugging installation problems: https://www.overleaf.com/ For book-length or thesis-length projects, it is worth installing LaTeX on your own machine.
Students have reported that the online help on Overleaf has been very useful to them. I also provide some tutorial information for them in https://leanpub.com/design_report_guidelines
A absolutely recommend a text-only format and to store it along with the source code in version control.
I've used reStructuredText for smaller documentations and LaTeX for larger ones. (I also used the LEO text editor to document a large library in HTML/chm, it is interesting but this leans more to the experimental side).
reStructuredText has some specific support which makes it more powerful than Markdown, for example it has extensive support for formatted code listings, and can add tables of content. It has many output options. You can convert it to wikitext, using the pandoc vonversion utility. Also, github can render it. There is a python pip package called restview which renders your document in the browser, including links, while you type it - this works very smoothly.
With LaTeX, code listings (I guess you don't need them here) be done using the "minted" Package. What makes LaTeX extremely good specifically for the purpose of documentation is, apart from a very consistent formatting, its support for a keyword index. And this is something you really need to make information discoverable in a large documentation. It is actually fun to build it. You can compile documents to different paper sizes, one in B5 which fits well as a PDF on a medium screen, and one for printing on A4 paper, for example. With the right font settings, it looks really really good and top notch readable.
Book recommendation (and a good example on how to do it well) : Leslie Lamport, LaTeX, A Document Preparation System.
Edit: Also, I have good experiences with inkscape for graphics. Readers love graphics.
Edit: Racket's scribble has been mentioned. I think it is good.