doc: add issue templates

Added the following templates:
- Bug Report
- Feature Request
- Documentation Improvement

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve by fixing bugs
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Share the config: `Please don't share your api_hash & api_id`
+```yaml
+chat_id: telegram_chat_id
+last_read_message_id: 0
+media_types:
+- audio
+- photo
+- video
+- document
+- voice
+file_formats:
+  audio:
+  - all
+  document:
+  - all
+  video:
+  - all
+```
+
+**Python Version**
+Python: [e.g. 3.7.7]
+
+**OS:**
+The OS and its version: [e.g. Ubuntu 20.04]
+
+**Logs**
+Logs showing the exception
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/documentation-improvement.md

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+---
+name: Documentation Improvement
+about: Report wrong or missing documentation.
+title: 'DOC:'
+labels: ''
+assignees: ''
+
+---
+
+#### Location of the documentation
+
+[this should provide the location of the documentation, e.g. "CONTRIBUTION.md" or the URL of the documentation, e.g. "https://github.com/Dineshkarthik/telegram_media_downloader/blob/master/CONTRIBUTING.md"]
+
+#### Documentation problem
+
+[this should provide a description of what documentation you believe needs to be fixed/improved]
+
+#### Suggested fix for documentation
+
+[this should explain the suggested fix and **why** it's better than the existing documentation]

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.