Update footer year and improve semantic HTML

[sanjana2505006]

Changes I made:

• The website footer displayed the copyright range as © 2005-2025. Since we are now in 2026, this is technically outdated.
  I updated the text to © 2005-2026 in index.html, download.html and 404.html

• The <b> tag is a "presentational" element. It tells the browser how it should look (bold), but not what it is. Screen readers often treat it as normal text. So I replaced these with <strong> tags.

This PR fixes issue #21

[jamesfredley]

@sanjana2505006 These pages are created and pushed to this repository from https://github.com/apache/grails-static-website. Please make these adjustments on that repository with a PR. https://github.com/apache/grails-static-website/blob/61005d16ea315116f50b488b18075e6f91cf9816/templates/document.html#L152

Separately the readme for this repository should include details on the source for the website and documentation from https://github.com/apache/grails-static-website and https://github.com/apache/grails-core.

Thank you for your continued contributions. There have not historically been many contributions for the website, so a lot of detail needs to be added to the readme's. Sorry this was not in place when you began making contributions.

[sanjana2505006]

Hello @jamesfredley, Thank you for the clarification and for taking the time to explain how these pages are managed.

That makes sense, I wasn’t aware that the website content is generated from the grails-static-website repository. I’ll move the changes there and open a PR in the appropriate place.
I also appreciate the additional context around the README and the documentation sources. It’s helpful to understand the structure better, and I’ll keep that in mind for future contributions as well.

[jamesfredley]

This PR will be replaced with a similar PR against https://github.com/apache/grails-static-website