chore: change Gitter to Discord throughout project.

[Koooooo-7]

Summary
According to #1224, make changes to separated PRs.
Change Gitter to Discord throughout project firstly.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Repo settings
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

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✅ Preview: https://docsify-preview-git-change-to-discord.docsify-core.now.sh

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[vercel]

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[Koooooo-7]

Vercel deployed faild cuz timeout , I think it's unnecessary tho.