.github: add issue templates

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: New issue for reproducible bug
+about: If you found a reproducible bug, submit it along with as much info as possible.
+---
+
+*Please include as much info as possible to save me (solo maintainer helping for free) some time. A [minimal, complete, and reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) is a must. Link to a gist if you don't feel like posting all the code inline. At the same time, please leave out unnecessary code so I don't need to wade through a hundred lines to get to the problematic part. Tell me your OS, FFmpeg version, etc. if there's even a slim chance of relevancy.*

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+
+contact_links:
+- name: Questions about usage
+  url: https://github.com/zmwangx/rust-ffmpeg/discussions
+  about: If you have a question about usage, please use discussions instead of opening a (non-)issue. Note that I (solo maintainer short on time) unfortunately might not be able to respond, though I try to be helpful when time permits.

.github/ISSUE_TEMPLATE/feature.md

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+---
+name: New feature request
+about: If you have a feature request, your best bet is probably a PR. However, for anything nontrivial, please open an issue to discuss it first.
+---
+
+*Please discuss your new feature before implementing it if it's nontrivial. Adding a small method is probably trivial, anything larger than that, maybe not. Note that API stability is paramount.*