Docs: Open Specification link in new tab

[melissahenderson]

The Starlight framework now supports opening sidebar links in a new tab: https://starlight.astro.build/guides/sidebar/#custom-html-attributes

Configure each of the following, in the documentation sidebar, to open in a new tab

• Specification
• Resources > External Resources > Interledger
• Resources > External Resources > Open Payments API
• Resources > External Resources > Payment Handler API
• Resources > External Resources > Payment pointers
• Resources > External Resources > Payment Request API

[melissahenderson]

I'm unable to run the WM docs locally because of an error that HJ will look in to, but this issue is completed in my own branch. I'll push when I'm able to test my updates.