doc: update interface, –stdin flag, IPC-command signtx (#31005) #33765

1. 
   cobratbq commented at 11:24 pm on November 2, 2025: none

   Updates to documentation for External Signer.

   • Added mention that signtransaction command is no longer primary mechanism.
   • Document IPC mechanism via --stdin flag followed with stdin-content.
   • Document signtx IPC-command followed by Base64-encoded PSBT.
2. doc: update interface, --stdin flag, IPC-command signtx 6f5fc6a949
3. DrahtBot added the label Docs on Nov 2, 2025
4. 
   DrahtBot commented at 11:25 pm on November 2, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33765.

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