Add Prerequisite section to the Blank Issue Template

[mocodesss]

Overview

We need to directly clarify who should be assigning themselves to issues by adding a Prerequisite section to the top of the Blank Issue template. This prevents contributors who haven't completed onboarding from self-assigning issues, which helps keep issues available to active members and reduces cleanup/re-prioritization work for the merge team and dev leads.

Action Items

• Open the .github/ISSUE_TEMPLATE/blank-issue-template.md file.
• Add a ###Prerequisite subheading above ###Overview
• Add this text to that new section:

1. Be a member of Hack for LA. (There are no fees to join.) If you have not joined yet, please follow the steps on our [Getting Started page](https://www.hackforla.org/getting-started) and attend an onboarding session.
2. Before you claim or start working on an issue, please make sure you have read our [How to Contribute to Hack for LA Guide](https://github.com/hackforla/website/blob/7f0c132c96f71230b8935759e1f8711ccb340c0f/CONTRIBUTING.md).

• Test the template update by following the instructions outlined here: Create wiki page to document test procedure for issues that modify issue templates #4786 (comment) (Be sure to include your URL from testing in your PR).

Resources/Instructions

• refer to .github/ISSUE_TEMPLATE/add-githib-handle.md as an example of an issue template .md file with a ###Prerequisite section. Here is a link to what that looks like in Github: https://github.com/hackforla/website/issues/new?template=add-github-handle.md
• GitHub's template documentation- General information about templates; note we use .md files for our templates.
• Context for how H4LA uses issue templates: https://github.com/hackforla/website/wiki/How-to-create-issues#step-2-choose-the-right-template