Improve supportability of the PHP agent

[estolfo]

This issue tracks changes we would like to make to the PHP agent and its documentation that would improve the onboarding and troubleshooting experience.

• [ ] Video/tutorial could be helpful - “Quick start”
• [ ] Reduce default error level for stderr to error, not critical
• [ ] Message about the apm server url being a default value
• [ ] Improve troubleshooting section of the documentation
• [ ] Steps on how to configure and get logs from the agent
• [x] Proactive check for file permissions that could prevent the agent to load - https://github.com/elastic/apm-agent-php/pull/1044