Move troubleshooting/Q&A to "Discussions"

- Makes this clearer in README.md
- Removes the "troubleshooting" issue template
- Adds a .github/ISSUE_TEMPATE/config.yml that disables blank issues and
  directs non-bug, non-feature-request discussions to the Discussions
  forum

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+- name: Troubleshooting, etc.
+  url: https://github.com/jsvine/pdfplumber/discussions
+  about: Use 'Discussions' to request assistance, ask questions, etc.

.github/ISSUE_TEMPLATE/troubleshooting.md

@@ -1,51 +0,0 @@
----
-name: Troubleshooting
-about: Ask for assistance in using pdfplumber
-title: ''
-labels: troubleshooting
-assignees: ''
----
-
-## What are you trying to do?
-
-*A clear and concise description of your goals.*
-
-
-## What code are you using to do it?
-
-*Paste it here, or attach a Python file.*
-
-
-## PDF file
-
-*Please attach the PDFs used in the code.*
-
-*If you need to redact text in a sensitive PDF, you can run it through [JoshData/pdf-redactor](https://github.com/JoshData/pdf-redactor).*
-
-
-## Expected behavior
-
-*What did you expect the result __should__ have been?*
-
-
-## Actual behavior
-
-*What actually happened, instead?*
-
-
-## Screenshots
-
-*If applicable, add screenshots to help explain your problem.*
-
-
-## Environment
-
-- pdfplumber version: [e.g., 0.5.22]
-- Python version: [e.g., 3.8.1]
-- OS: [e.g., Mac, Linux, etc.]
-
-
-## Additional context
-
-*Add any other context/notes about the problem here.*
-