Add Contributor Guidelines, User Guide, and Pull Request Template

[IanHoang]

Description

For the last few months, there's been some confusion about contributing to the workloads repository. This PR streamlines the process and adds details related to how to contribute to the workloads repository, establishes new PR best practices, and how to backport changes appropriately.

Issues Resolved

#419
opensearch-project/opensearch-benchmark#583 (comment)

Backport to Branches:

• 6
• 7
• 1
• 2
• 3

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[OVI3D0]

IMO this guide is very useful, and would really help someone who is new to the workloads repository (like myself) to get up and running without much confusion. Thanks for this!

[opensearch-trigger-bot]

The backport to 7 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-7 7
# Navigate to the new working tree
pushd ../.worktrees/backport-7
# Create a new branch
git switch --create backport/backport-425-to-7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6c2d9cb46ce8125148dbb67ed8c1e752cad53e6b
# Push it to GitHub
git push --set-upstream origin backport/backport-425-to-7
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-7

Then, create a pull request where the base branch is 7 and the compare/head branch is backport/backport-425-to-7.