docs: Extended developer guideline doc

[thompson-tomo]

Closes #2206

This focuses on providing a detailed developer guide which covers adding, deprecating and removal of gems including the steps needed.

[xuan-cao-swi]

A couple of thoughts:

• Do we actually need both instrumentation/CONTRIBUTING.md and the main CONTRIBUTING.md?
  It might make sense to merge them into a single document and include a table of contents for easier navigation. (example)
• Similarly, README.md and datadog-porting-guide.md seem to overlap in purpose. Maybe they could be consolidated or reorganized into a more unified structure.

[thompson-tomo]

I agree about merging the contributing guides but would rather treat them separately to avoid making this PR too big.

In relation to dd migration guide, I am inclined to leave it as is as I suspect it would not often be read. I will think if it can be better incorporated.