Validate and improve draft coding conventions

[fdesbiens]

I committed draft coding conventions on the coding-conventions branch. The file is here:
https://github.com/eclipse-threadx/threadx/blob/coding-conventions/CODINGCONVENTIONS.md

Right now, the content simply reflects what is found in our quality manual, which belongs to our safety artefacts package.

@eclipse-threadx/iot-threadx-committers and @eclipse-threadx/iot-threadx-contributors: Please review the C coding standards and propose entries for the assemblyt coding standards (section 2,)

[danutza77]

Here's my feedback:

1. C coding conventions
   Versions of ThreadX 6.0 and greater follow the MISRA-C:2004 and MISRA C:2012 coding conventions.

--> As a note, the MISRA-C:x guidelines should be made available to the project team
I'll take this as an action: to discuss with Wayne and see if we can find a solution

3.0. File headers
ThreadX file headers are m...

--> Just for consistency, could we please make "3.0." into "3." like for the rest of the chapters?

Thank you,
Dana