docs: reorganized behavioral experiment runner doc

[musslick]

Description

The documentation for running behavioral experiments was scattered across sections, and it was also not clear how the different components and subpackages relate to one another. Furthermore, the doc structure was obscure as to whether AutoRA can support other types of automated data collection.

The following restructuring is an attempt to fix these issues:

- Behavioral Web Experiments:
    - Home: 'experiment-runner/behavioral/index.md'
    - Example: 'experiment-runner/behavioral/example.md'
    - Using Firebase: 'experiment-runner/behavioral/firebase.md'
    - Firebase-Prolific Runner: '!import https://github.com/autoresearch/autora-experiment-runner-firebase-prolific/?branch=main&extra_imports=["mkdocs/base.yml"]'
    - Firebase Experimentation Manager: '!import https://github.com/autoresearch/autora-experiment-runner-experimentation-manager-firebase/?branch=main&extra_imports=["mkdocs/base.yml"]'
    - Prolific Recruitment Manager: '!import https://github.com/autoresearch/autora-experiment-runner-recruitment-manager-prolific/?branch=main&extra_imports=["mkdocs/base.yml"]'

Type of change

• docs: Documentation only changes

Remarks (Optional)

• the PR is not intended to complete the runner doc but merely to restructure it. That said, the behavioral experiment runner doc could likely be improved further.