templates: add issue and PR templates for repo (#61)

* add issue and PR templates for repo

Signed-off-by: Calvin Weng <wenghao@pingcap.com>

.github/ISSUE_TEMPLATE/change-request.md

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+---
+name: "\U0001F680 Change Request"
+about: Any suggestion? Want to add something new?
+---
+
+## Change Request
+
+<!--This repository is ONLY used to solve issues related to TiKV Website/Docs.
+For other issues (related to TiKV, PD, etc), please move to [other repositories](https://github.com/tikv).-->
+
+### Describe the problem
+
+- Relevant URL(s): <URL>
+
+<!-- Provide details for typos, grammatical errors, terminology misuse, ambiguity, broken links, missing info, etc. -->
+
+### Suggest an improvement/fix
+
+<!--If you have specific ideas about how we can fix this, let us know. -->

.github/ISSUE_TEMPLATE/question.md

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+---
+name: "\U0001F914 Question"
+about: Usage question that isn't answered in docs or discussion
+---
+
+## Question
+
+<!--This repository is ONLY used to solve issues related to TiKV Website/Docs.
+For other issues (related to TiKV, PD, etc), please move to [other repositories](https://github.com/tikv).-->
+
+Before submitting your question, make sure you have:
+
+- Searched existing Stack Overflow questions.
+- Googled your question.
+- Searched the open and closed [GitHub issues](https://github.com/tikv/website/issues).
+
+Now, please describe your question here:

.github/pull_request_template.md

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+<!--Thanks for your contribution to TiKV documentation. -->
+
+### What is changed?
+
+<!--Tell us what you did and why.-->
+
+### Any related PRs or issues?
+
+<!--Provide a reference link that is related to your change. -->
+
+### Which version does your change affect?