docs(readme): revamp overview, quick start, and contribution links

[Richin]

Rewrite README.md to provide a clearer overview of CodeIgniter 4, including key framework features, requirements, and community resources. Add a pragmatic quick-start section with Windows-specific notes, highlight common spark/Composer workflows, and outline the project structure so newcomers can get productive quickly. Clarify contribution and security guidance by linking to the relevant docs.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [x] Conforms to style guide

[mergeable[bot]]

Hi there, Richin! :wave:

Thank you for sending this PR!

We expect the following in all Pull Requests (PRs).

• PRs must be sent to the appropriate branch
• All git commits must be GPG-signed
• Must follow our style guide
• Be commented in the PHP source file
• Be documented in the user guide
• Be unit tested
• Pass all checks in GitHub Actions

[!IMPORTANT] We expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.

If pull requests do not comply with the above, they will likely be closed. Since we are a team of volunteers, we don't have any more time to work on the framework than you do. Please make it as painless for your contributions to be included as possible.

See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md

Sincerely, the mergeable bot 🤖

[paulbalandan]

Please be aware that you are editing the README applicable for the development repository. If you are intending to revamp the README for the framework, you might want to go to admin/framework/README.md instead.