New issue templates

[tnytown]

In this changeset, I've cribbed some issue templates from GitHub's issue template templates. I also added links to pip-audit's issue tracker and the troubleshooting section in our README.

Here's how the changes look:
https://github.com/tnytown/gh-action-pip-audit/issues/new/choose

I don't love how the feature request template is worded, but I also don't know how I would change it. Suggestions welcome!

Resolves #31.

[woodruffw]

Looks good on first blush, two thoughts:

1. Let's add a warning (similar to the one in pip-audit's bug template) saying that we'll close or deprioritize reports that don't fill in all of the report's sections.
2. Let's add a notice reminding people to report bugs to pip-audit rather than this repo, if the buggy behavior in question appears to be solely in pip-audit rather than in this action's scaffolding.

For reference: https://github.com/pypa/pip-audit/issues/new?assignees=&labels=bug-candidate&template=bug_report.md&title=

[tnytown]

Let's add a warning (similar to the one in pip-audit's bug template) saying that we'll close or deprioritize reports that don't fill in all of the report's sections.

Hm, do we want to use issue forms? We can split the issue filing GUI into multiple fields and require input for each of those fields.

[woodruffw]

Hm, do we want to use issue forms? We can split the issue filing GUI into multiple fields and require input for each of those fields.

Yes! I've seen these around, but I didn't know the right set of keywords to search to find the docs for them. We should absolutely use these.

[tnytown]

I've updated the forms in my fork! How do they look?

[woodruffw]

LGTM, thanks!