Improve dependencies.md to document IPC dependencies better:
- Link to native_capnp.mk file not capnp.mk file so it's possible to see what
version of Cap'n Proto is being used in release binaries. This is important
since #31895 dropped the "Version Used" column and the capnp.mk file does not
include version number.
- Indicate Capn"Proto is used for IPC and link to multiprocess.md documenting
the feature.
- Link to correct PR requiring Cap'n Proto 0.7.1. Previous link was
pointing at PR which required 0.7.0.
- Mention libmultiprocess as a dependency even though it is included as a git
subtree and can be built as a cmake subproject. Libmultiprocess still needs
to be built separately when cross compiling, and is useful to build separately
when developing, and is still a depends package.
Based on 2cf352fd8e6a77003e38d954b6c879b20d4b960a from #33623 by willcl-ark
which made similar changes in the 29.x branch.