feat: add markdown templates

templates/Welcome-to-the-AFFiNE-Alpha.md

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+The AFFiNE Alpha is here! You can also view our [Official Website](https://affine.pro/)!
+
+**What's different in AFFiNE Alpha?**
+
+1.  A much ~smoother editing~ experience, with much ~greater stability~;
+2.  More complete ~Markdown~ support and improved ~keyboard shortcuts~;
+3.  New features such as ~dark mode~;
+    - **Switch between view styles using the** ☀ **and** 🌙.
+4.  ~Clean and modern UI/UX~ design.
+
+**Looking for Markdown syntax or keyboard shortcuts?**
+
+- Find the (?) in the bottom right, then the ️⌨️, to view a full list of `Keyboard Shortcuts`
+
+**Have an enjoyable editing experience !!!** 😃
+
+Have some feedback or just want to get in touch? Use the (?), then 🎧 to get in touch and join our communities.
+
+**Still in development**
+
+There are also some things you should consider about this AFFiNE Alpha including some ~limitations~:
+
+- Single page editing - currently editing multiple docs/pages is not supported;
+- Changes are not automatically stored, to save changes you should export your data. Options can be found by going to the top right and finding the `⋮` icon;
+- Without an import/open feature you are still able to access your data by copying it back in.
+
+**Keyboard Shortcuts:**
+
+1.  Undo: `⌘+Z` or `Ctrl+Z`
+2.  Redo: `⌘+⇧+Z` or `Ctrl+Shift+Z`
+3.  Bold: `⌘+B` or `Ctrl+B`
+4.  Italic: `⌘+I` or `Ctrl+I`
+5.  Underline: `⌘+U` or `Ctrl+U`
+6.  Strikethrough: `⌘+⇧+S` or `Ctrl+Shift+S`
+7.  Inline code: `⌘+E` or `Ctrl+E`
+8.  Link: `⌘+K` or `Ctrl+K`
+9.  Body text: `⌘+⌥+0` or `Ctrl+Shift+0`
+10. Heading 1: `⌘+⌥+1` or `Ctrl+Shift+1`
+11. Heading 2: `⌘+⌥+2` or `Ctrl+Shift+2`
+12. Heading 3: `⌘+⌥+3` or `Ctrl+Shift+3`
+13. Heading 4: `⌘+⌥+4` or `Ctrl+Shift+4`
+14. Heading 5: `⌘+⌥+5` or `Ctrl+Shift+5`
+15. Heading 6: `⌘+⌥+6` or `Ctrl+Shift+6`
+16. Increase indent: `Tab`
+17. Reduce indent: `⇧+Tab` or `Shift+Tab`
+
+**Markdown Syntax:**
+
+1.  Bold: `**text**`
+2.  Italic: `*text*`
+3.  Underline: `~text~`
+4.  Strikethrough: `~~text~~`
+5.  Inline code: `` `text` ``
+6.  Heading 1: `# text`
+7.  Heading 2: `## text`
+8.  Heading 3: `### text`
+9.  Heading 4: `#### text`
+10. Heading 5: `##### text`
+11. Heading 6: `###### text`