Add `to_html` shard to HTML Builder section

[sbsoftware]

Link

https://github.com/sbsoftware/to_html.cr

Note: The repo itself is quite young because the code has been extracted from another project. I am using this shard in production builds regularly.

Checklist

• [x] - Shard is at least 30 days old.
• [x] - Shard has CI implemented.
• [x] - Shard has daily/weekly periodic builds (ideally with Crystal latest and nightly).