Merge bitcoin/bitcoin#28978: doc: Add multiprocess design doc

91dc48c doc: Add multiprocess design doc (Ryan Ofsky)

Pull request description:

  Add multiprocess design doc and existing multiprocess documentation into design and usage sections.

  Links to rendered markdown:

  https://github.com/ryanofsky/bitcoin/blob/pr/ipcdoc/doc/design/multiprocess.md
  https://github.com/ryanofsky/bitcoin/blob/pr/ipcdoc/doc/multiprocess.md

  ---

  This PR is part of the [process separation project](bitcoin/bitcoin#28722).

ACKs for top commit:
  fjahr:
    ACK 91dc48c
  achow101:
    ACK 91dc48c
  TheCharlatan:
    ACK 91dc48c
  stickies-v:
    ACK 91dc48c - left a couple of improvements but agreed that iterating in future PRs is better.

Tree-SHA512: 8890abd85555eb3a64e75e3393f867839771a83aaba7667c19eccac2e959fb37b13c3bc1906ff06ff3d66609e2c72835b4b9a22d31e997af4489092418eeb001

Author: achow101