Add pull request template to repo, with contribution checklist

[spwoodcock]

• After orthophoto merge: parallelize the block loop (in-order writer) #2035 I thought it would be wise to add a pull request template to the repo.
• Each time a new PR is made, some text will be prefilled, including a checklist to ensure the contributors guide has been read and agreed to.
• Edit I also added a section to provide test datasets, run logs, other info --> something we so commonly have to ask people to provide after the PR is made =)

Example rendering of template below:

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Related Issue

Fixes #

Describe this PR

A brief description of what this changes and why.

AI Tool Usage

• No AI tools were used
• AI tools were used (complete below)

If AI-assisted:

• Tool(s) used:
• What was generated:
• What you reviewed and changed:

Checklist

• I have read and agree to the Contributing Guide
• PR is focused and small
• I understand all code in this PR and can answer questions about it
• No secrets, credentials, or sensitive data are included
• Commit messages are descriptive

Testing

• Test dataset(s) used (link to source, e.g. ODMdata or a public download):
• Command(s) run:
• Logs from successful ODM run(s) (attach as file, gist, or paste relevant excerpt):
• Notes on results (screenshots of outputs, comparisons before/after, anything reviewers should look at):