fix: update PR template and CONTRIBUTING guide to GitHub Flow

[gl3nng0uld]

Proposed changes

From the recent development activity, I found that the project adopts the GitHub Flow: branching off from main, and then merge in main again.

Instead, both the PR template .github/pull_request_template.md and the CONTRIBUTING.md guide say to create a new branch from develop and to never do a pull request directly on main.

Am I wrong? If so, please close this PR.

Fixes

n/a

Types of changes

What types of changes does your code introduce to agents-aea?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes and CI passes too
• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease.
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

n/a