chore: Add description templates

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

.github/ISSUE_TEMPLATE/bug_report.md

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+<!--
+Thank you for sending a bug report! Here some tips:
+
+1. Please fill the template below to make it easier to debug your problem.
+2. If you are not sure is it a bug or not, you can ask your question in the Kubernetes slack channel `#dexidp`
+-->
+
+**Expected behavior (what you expected to happen)**:
+
+**Actual behavior (what actually happened)**:
+
+**Steps how to reproduce it**:
+1. ...
+2. ...
+3. ...
+
+**Anything else we need to know?**:
+
+**Environment**:
+- Dex version:
+- Storage type:
+- Installation type (official docker image, helm chart, etc.):
+
+**Additional information for debugging (is necessary)**:
+- Dex configuration:
+- Dex logs during the problem:

.github/ISSUE_TEMPLATE/config.yaml

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+blank_issues_enabled: false
+contact_links:
+- name: Dex Slack channel
+  url: https://kubernetes.slack.com/messages/dexidp
+  about: Please ask and answer questions here.

.github/ISSUE_TEMPLATE/feature_request.md

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+<!--
+ Thank you for sending a feature request! 
+ Please describe in detail what do you want and why by filling the template below.
+ -->
+
+**What would you like to be added/changed**:
+
+**Why we need this**:

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for sending a pull request! Here some tips for contributors:
+
+1. Fill the description template below.
+2. Sign a DCO (if you haven't already signed it).
+3. Include appropriate tests (if necessary). Make sure that all CI checks passed.
+4. If the Pull Request is a work in progress, make use of GitHub's "Draft PR" feature and mark it as such.
+-->
+
+**Overview**:
+<!-- Describe your changes briefly here. -->
+
+**What problem does it solve?**:
+<!--
+- Please state in detail why we need this PR and what it solves.
+- If your PR closes some of the existing issues, please add links to them here.
+  Mentioned issues will be automatically closed.
+  Usage: "Closes #<issue number>", or "Closes (paste link of issue)"
+-->
+
+**Special notes for a reviewer**: