Customise Starlight out-the-box components

[huijing]

This issue helps keep track of https://github.com/withastro/starlight/issues/415 and its corresponding PR https://github.com/withastro/starlight/pull/709, which is highly relevant to our docs sites because we do customise the site header via https://gist.github.com/giuseppelt/7f918a3ac02a011d76811ae472f8bf09. This is known to be a hacky way of doing it, and if the ability to customise Starlight components becomes a first-party solution, we can update our implementations accordingly.

Depending on what the final API is, we may or may not be able to add those custom Starlight components to our shared npm library.

[huijing]

We have updated all our documentation sites to use the proper API:

• https://github.com/interledger/open-payments/pull/300/
• https://github.com/interledger/rafiki/pull/2010
• https://github.com/WICG/webmonetization/pull/399
• https://github.com/interledger/paymentpointers.org/pull/35
• https://github.com/interledger/interledger.org-developers/pull/17