Dev guide update

On top of being a central place to retrieve information on how to develop and maintain a PyAnsys project, the dev guide can also be used through AI tooling, e.g. the us of copilot-instructions.md presented recently by @Revathyvenugopal162

Following that discussion, we could have a look at the repo and see what should be added / removed / updated.

I plan to do that while performing the OSS's technical review for a pyaedt toolkit. Any help is welcome !

Here is a list (to be updated...):

Start by reviewing the existing PyAnsys developer guide repository and the discussion about copilot-instructions.md. Inventory what should be added, removed, or updated, then confirm the guide reflects the agreed development and maintenance guidance and supports the intended AI-tooling use cases.

Written by the indexing model from the issue text.