Update cloud experiment documentation

[ardier]

This pull request updates the FuzzBench documentation to provide clearer and more comprehensive instructions for running and submitting experiments, especially when contributing via pull requests. The main focus is on clarifying the workflow differences, experiment naming requirements, and artifact submission process for external contributors.

Documentation improvements for experiment submission:

• Added a new "Submitting Experiments via Pull Requests" section to running_an_experiment.md, detailing the workflow, experiment name requirements, artifact submission, and log access for contributors submitting experiments through GitHub PRs.
• Updated adding_a_new_fuzzer.md to reference the new instructions for submitting experiments via pull requests, improving discoverability and guidance for new contributors.

General documentation enhancements:

• Expanded the introduction to run_experiment.py in running_an_experiment.md to explain its purpose and workflow, helping users understand how experiments are started and managed in FuzzBench.

[ardier]

Here is the associated issue.