Add Bug report and Feature request templates (#3471)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a problem to help us improve
+---
+
+<!--
+
+Thanks for taking an interest in Scrapy!
+
+If you have a question that starts with "How to...", please see the Scrapy Community page: https://scrapy.org/community/.
+The Github issue tracker's purpose is to deal with bug reports and feature requests for the project itself.
+
+Keep in mind that by filing an issue, you are expected to comply with Scrapy's Code of Conduct, including treating everyone with respect: https://github.com/scrapy/scrapy/blob/master/CODE_OF_CONDUCT.md
+
+The following is a suggested template to structure your issue, you can find more guidelines at https://doc.scrapy.org/en/latest/contributing.html#reporting-bugs
+
+-->
+
+### Description
+
+[Description of the issue]
+
+### Steps to Reproduce
+
+1. [First Step]
+2. [Second Step]
+3. [and so on...]
+
+**Expected behavior:** [What you expect to happen]
+
+**Actual behavior:** [What actually happens]
+
+**Reproduces how often:** [What percentage of the time does it reproduce?]
+
+### Versions
+
+Please paste here the output of executing `scrapy version --verbose` in the command line.
+
+### Additional context
+
+Any additional information, configuration, data or output from commands that might be necessary to reproduce or understand the issue. Please try not to include screenshots of code or the command line, paste the contents as text instead. You can use [GitHub Flavored Markdown](https://help.github.com/en/articles/creating-and-highlighting-code-blocks) to make the text look better.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for an enhancement or new feature
+---
+
+<!--
+
+Thanks for taking an interest in Scrapy!
+
+If you have a question that starts with "How to...", please see the Scrapy Community page: https://scrapy.org/community/.
+The Github issue tracker's purpose is to deal with bug reports and feature requests for the project itself.
+
+Keep in mind that by filing an issue, you are expected to comply with Scrapy's Code of Conduct, including treating everyone with respect: https://github.com/scrapy/scrapy/blob/master/CODE_OF_CONDUCT.md
+
+The following is a suggested template to structure your pull request, you can find more guidelines at https://doc.scrapy.org/en/latest/contributing.html#writing-patches and https://doc.scrapy.org/en/latest/contributing.html#submitting-patches
+
+-->
+
+## Summary
+
+One paragraph explanation of the feature.
+
+## Motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+## Describe alternatives you've considered
+
+A clear and concise description of the alternative solutions you've considered. Be sure to explain why Scrapy's existing customizability isn't suitable for this feature.
+
+## Additional context
+
+Any additional information about the feature request here.