docs: refine README

[lechnerc77]

Purpose

• Refinement of README
• Start of Developer Guideline
• Alignment of Envionment Parameters

Does this introduce a breaking change?

[X] Yes - The environment parameter `BTP_CLIENT_SERVER` was changed to `BTP_CLI_SERVER`
[ ] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[X] Other... Please describe: Alignment of parameters

How to Test

n/a

What to Check

n/a

Other Information

n/a

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• [X] The PR is assigned to the Project board and a status is set (typically "in review").
• [X] The PR has the matching labels assigned to it.
• [X] The PR has a milestone assigned to it.
• [X] If the PR closes an issue, the issue is referenced.
• [X] Possible follow-up issues are created and linked.