Add the purpose section to the pull request template

[Tharindu37]

To suggest a change to the Handbook, please review the Handbook Guidlines section.

Please provide a brief explanation for this change:

Add the purpose section to the pull request template

Preview Link: https://pr-20-sef-handbook.surge.sh/

Please indicate the types of revisions being suggested for the Handbook (please check all that apply):

• [x] Small improvement (typos, clarifications, etc.)
• [ ] Adding a new section
• [ ] Modifying existing section
• [ ] Documenting a new process
• [ ] Adding a new page or directory
• [ ] Other

Author Checklist

• [x] Provided a concise title for the PR
• [ ] Provided a brief explanation for this change (Say why not just what)
  • (Attach screenshots, Slack conversations, etc. as necessary)
• [ ] Indicated the types of changes included in this PR
• [ ] Verified that no confidential data is in this PR
• [ ] Posted an update in #handbook channel
• [ ] If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #general channel