[DOCS] Add Java snippets to English guides

[mkurapov]

The Java Open Payments snippets were done as part of the following PR:

https://github.com/interledger/open-payments/pull/745/files

However, the snippets still need to be completed for the "Guides" section.

The following guides need to be completed:

• accept-otp-online-purchase
• onetime-remittance-fixed-debit
• onetime-remittance-fixed-receive
• recurring-subscription-incoming-amount
• recurring-remittance-fixed-debit
• recurring-remittance-fixed-receive
• split-payments
• outgoing-grant-future-payments

See: https://openpayments.dev/guides/accept-otp-online-purchase/