Docs: Add Development Quickstart for Linux Users

[menschel]

Add a comprehensive and straight-forward guide
how to build the project on Linux.
The focus until now was only on Windows.

[jlpoltrack]

Thanks for submitting, a few general thoughts and suggestions:

• No commands here look Linux specific to me :), e.g. git, python and pio are all cross platform.
• The note about git clone recursive isn't necessary as the run_setup already does this: https://github.com/olliw42/mLRS/blob/main/run_setup.py#L65-L70 so I think this could be removed.
• Installing the necessary python modules needed is a problem at the moment, however other solutions have been proposed so while this will work today this will also likely change in the future. See corresponding PRs: Add requirements.txt to track required python modules mLRS#244 Fix issue with run_setup and dronecan mLRS#228
• The PIO build commands are helpful but would be specific to building for ESP targets only (not STM32 which is the preferred MCU for mLRS), so perhaps it makes more sense to include these on the ESP development page instead.

[menschel]

* No commands here look Linux specific to me :), e.g. git, python and pio are all cross platform.

This projects build instruction is purely based on Windows and VSCode. Nobody in a sane state of mind uses VSCode on Linux. I remember when Microsoft tried to backdoor themselves into the Raspberry Pi Package Authority and I remember 2 decades where Microsoft was the Enemy of Open Source.

* The note about git clone recursive isn't necessary as the run_setup already does this: https://github.com/olliw42/mLRS/blob/main/run_setup.py#L65-L70 so I think this could be removed.

I try to follow best practices.

* Installing the necessary python modules needed is a problem at the moment, however other solutions have been proposed so while this will work today this will also likely change in the future.  See corresponding PRs: [Add requirements.txt to track required python modules mLRS#244](https://github.com/olliw42/mLRS/pull/244) [Fix issue with run_setup and dronecan mLRS#228](https://github.com/olliw42/mLRS/pull/228)

This has been taken care of dronecan/pydronecan#76 .

* The PIO build commands are helpful but would be specific to building for ESP targets only (not STM32 which is the preferred MCU for mLRS), so perhaps it makes more sense to include these on the ESP development page instead.

That has been done. I disagree with this project's focus on STM32. It was a pain worth of a Chuck Norris Joke to get Seeed WIO-E5 modules working with mLRS. Documentation has been neglected in this project.

[olliw42]

many thx for the PR, much appreciated.
I'd like to be honest. To me it reads now like written by someone way too savvy in these things going over the head of many. I am one of these many. What looked to me like a somewhat simple and straightforward procedure reads now scary.
I also don't like e.g. that installing a specific git tool like tortoise is suggested. I don't like then install instructions are unneccesarily infectious to someone's system as not anyone may like the tool. Especially when it does things like mangling with explorer.
So, what I would suggest is to use what we had but add a Windows section title, and add a Linux section below for your things to fill in.

@brad112358 how are you dealing with esp targets?

I disagree with this project's focus on STM32.

you are free and welcome to disagree, but it's a fact that it is focused on STM32, and it's not going to change.

Documentation has been neglected in this project.

you should understand that we feel that we spend quite a lot of time on documentation.