Add templates for github issues and PRs

Author: jnewbery

.github/ISSUE_TEMPLATE.md

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+This issue tracker is for issues with the Bitcoin Core project website at
+[bitcoincore.org](https://bitcoincore.org).
+
+[bitcoin.org](https://bitcoin.org) is not connected with or maintained by the
+Bitcoin Core project contributors. Issues with that website should be reported
+at [the bitcoin.org
+repository](https://github.com/bitcoin-dot-org/bitcoin.org/issues).
+
+General bitcoin questions and/or support requests are best directed to the
+Bitcoin StackExchange at https://bitcoin.stackexchange.com.

.github/PULL_REQUEST_TEMPLATE.md

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+Use this page to open Pull requests for changes to the Bitcoin Core project
+website at [bitcoincore.org](https://bitcoincore.org).
+
+[bitcoin.org](https://bitcoin.org) is not connected with or maintained by the
+Bitcoin Core project contributors. Pull requests for changes to that website
+should be opened at [the bitcoin.org
+repository](https://github.com/bitcoin-dot-org/bitcoin.org/pulls).
+
+General bitcoin questions and/or support requests are best directed to the
+Bitcoin StackExchange at https://bitcoin.stackexchange.com.