Add development agreements documentation

[drew2a]

As demonstrated in #7799, there is currently a lack of clarity regarding our processes and the agreements we have made. To address this and ensure the transfer of knowledge to future team members, I have created the agreements.rst file. This file will serve as a central repository for listing all our agreements.

I propose a single rule for this document: any changes must receive approval through pull requests from all current developers. This ensures collective agreement and maintains the integrity of our processes.

To initiate the agreements.rst with relevant content, I have included our consensus on the branching model, as discussed in the following issues:

• Improvement request: Branching model #5569
• Improvement request: Branch instead of Fork #5575

The importance of having such a document, particularly in a distributed team with considerable turnover, cannot be overstated. Documenting agreements:

• Facilitates Onboarding: New developers can quickly familiarize themselves with the team's methodologies, significantly reducing the learning curve.
• Preserves Institutional Knowledge: With high turnover (the hight turnover of Tribler's developers has been shown here: Journal drew2a/ivory-tower#1 (comment)), valuable knowledge can be lost. A documented set of agreements helps preserve this knowledge, ensuring continuity and stability in the team's operations.
• Aids in Conflict Resolution: Having a reference point for previously made agreements can significantly reduce misunderstandings and conflicts within the team, as it provides a basis for resolution and decision-making (see also: Tie Breaker #7807).
• Ensures Continuous Improvement: By requiring unanimous approval for changes, the document becomes a living entity that evolves with the team, continually improving and adapting to new challenges and insights.

[drew2a]

As @qstokkink pointed out, the suggested file is a duplicate of an existing one: https://github.com/Tribler/tribler/blob/main/doc/contributing.rst