docs: refactor absolute links to relative anchors in README

[Aman071106]

PR Description

This PR updates the README by replacing external GitHub UI–dependent links (those containing ?tab=readme-ov-file) with clean internal relative anchors. These absolute links are fragile and can break if GitHub changes its interface or if a fork/mirror is used. Switching to internal anchors makes the documentation more stable, cleaner, and easier to maintain.

Summary of Changes

• Replaced links such as
  https://github.com/foss42/apidash?tab=readme-ov-file#mime-types-supported-by-api-dash-response-previewer
  with
  #mime-types-supported-by-api-dash-response-previewer
• Replaced repeated UI-based links in code generation, MIME-type listing, and feature description sections.
• Removed duplicate markdown lines caused by mixing external URLs and relative anchors.
• Ensures internal navigation works consistently across forks, mirrors, and offline viewers.

This improves the readability and reliability of the README without changing any functional content.

Related Issues

• For Documentation contributions are welcome! #527

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with the main branch before making this PR
• I am using the latest Flutter stable branch
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No - documentation-only update, tests not required.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux