chore: add web validator information to README

[davidgamez]

Summary:

This is part of the pre-release tasks. This PR adds the GTFS Web validator information to the project root's README file.

Expected behavior:

The GTFS Web Validator information is in the project root's README file and linking web client and service sub-packages README for more information.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[isabelle-dr]

Can we have the following order in the README.md:

• Introduction
• Using the web-based validator
• Using the Desktop app
• Using the command line
• Using Docker

Also, could you rename the video to validator_demo.mp4 and put it at the top below the introduction instead of the one we currently have showing the Desktop app?

[isabelle-dr]

We're missing a ":" after "two main components".

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1426 sources (~0 %) are corrupted.
Commit: 6697838
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.