Update issue template to break down by bug and template, add support doc to close #23133

Author: mdo

.github/ISSUE_TEMPLATE.md

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+Before opening:
+
+- [Search for duplicate or closed issues](https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue)
+- [Validate](https://html5.validator.nu/) and [lint](https://github.com/twbs/bootlint#in-the-browser) any HTML to avoid common problems
+- Read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md)
+
+Bug reports must include:
+
+- Operating system and version (Windows, Mac OS X, Android, iOS, Win10 Mobile)
+- Browser and version (Chrome, Firefox, Safari, IE, MS Edge, Opera 15+, Android Browser)
+- [Reduced test case](https://css-tricks.com/reduced-test-cases/) and suggested fix using [CodePen](https://codepen.io/) or [JS Bin](https://jsbin.com/)

.github/ISSUE_TEMPLATE/bug.md

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+Before opening:
+
+- [Search for duplicate or closed issues](https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue)
+- Read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md)
+
+Feature requests must include:
+
+- As much detail as possible for what we should add and why it's important to Bootstrap
+- Relevant links to prior art, screenshots, or live demos whenever possible

.github/ISSUE_TEMPLATE/feature.md

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+### Bug reports
+
+See the [contributing guidelines](CONTRIBUTING.md) for sharing bug reports.
+
+### How-to
+
+For general troubleshooting or help getting started:
+
+- Join [the official Slack room](https://bootstrap-slack.herokuapp.com/).
+- Chat with fellow Bootstrappers in IRC. On the `irc.freenode.net` server, in the `##bootstrap` channel.
+- Ask and explore Stack Overflow with the [`bootstrap-4`](https://stackoverflow.com/questions/tagged/bootstrap-4) tag.