Add Apache Iceberg to index.html, add alt text

[celestehorgan]

This commit adds the Apache Iceberg logo to the Storage and Infrastructure section of the website's index.html section. It also adds alt text to all logo files on the homepage.

The Iceberg and Orc logos are switched because there isn't enough padding pixels on the Iceberg logo so... this is easier! :D

[holdenk]

LGTM, if no one objects I'll merge this tomorrow probably.

[dongjoon-hyun]

To @holdenk and @celestehorgan , this PR seems to be manually crafting index.html file.

FYI, we are supposed to update the source file index.md first and include the re-generate HTML file. If the HTML file is the properly generated one already, what we need additionally is the missing change of the source file, index.md. Please include it to proceed further.

[celestehorgan]

@dongjoon-hyun Ahh, I see. Why are we committing the static generated html to begin with 🤔

I'll close this PR and re-open with an index.md edit.