[Docs] Fix broken markdown and incorrect info in contributing page

[yhk1038]

Problem

The contributing page (https://type-ruby.github.io/docs/project/contributing) has broken markdown rendering and incorrect community information.

1. Broken Markdown

In the "Report Bugs" section, there is a nested code block example that breaks markdown rendering:

**Example:**
\`\`\`markdown
## Bug: Type inference fails for array map

**T-Ruby Version:** v0.1.0
**Ruby Version:** 3.2.0

### Steps to Reproduce
\`\`\`trb
numbers: Array<Integer> = [1, 2, 3]
...
\`\`\`

The inner code block (trb) inside the outer code block (markdown) causes rendering issues.

2. Incorrect Community Information

The following incorrect information needs to be corrected:

Current (incorrect):

• "Discord - Real-time chat with community"
• "Twitter - @t_ruby"
• "Discord - Join our server"
• "Email - maintainers@t-ruby.org"

Should be:

• GitHub Issues for bug reports
• GitHub Discussions for questions and community discussion
• Website News for announcements
• Remove Discord, Twitter, and email references

Files to update

• docs/project/contributing.md

Required changes

1. Fix the nested code block in "Report Bugs" example section
2. Update "Where to Ask Questions" section:
   • Keep GitHub Discussions and GitHub Issues
   • Remove Discord reference
   • Remove Stack Overflow reference (optional)
3. Update "Contact" section:
   • Remove Twitter
   • Remove Discord
   • Remove or update email

[yhk1038]

Additional broken markdown found

Section: "3. Document the Types"

URL: https://type-ruby.github.io/docs/project/contributing#3-document-the-types

Same nested code block issue:

```markdown
### JSON

```trb
def parse_json(file: String): Hash<String, Any>
  JSON.parse(File.read(file))
end

The inner `trb` code block inside the outer `markdown` code block causes rendering issues.

### Summary of all broken markdown locations:

1. **"Report Bugs" section** - Example with nested markdown/trb blocks
2. **"3. Document the Types" section** - Example with nested markdown/trb blocks

[yhk1038]

Correction: Documentation Structure section is also affected

The nested code block issue in "3. Document the Types" section causes a cascading problem:

Because the markdown code block contains an inner trb code block, the closing backticks get misaligned. This causes the "Documentation Structure" section to also be rendered incorrectly - it gets absorbed into the broken code block context.

Affected sections (updated):

1. "Report Bugs" section - Nested markdown/trb blocks
2. "3. Document the Types" section - Nested markdown/trb blocks
3. "Documentation Structure" section - Cascading effect from Translation: All documents for ko, ja #2

The fix for #2 should also resolve #3.