Merge bitcoin/bitcoin#30741: doc: update documentation and scripts related to build directories

6a68343ffb doc: Prepend 'build/' to binary paths under 'src/' in docs (Lőrinc)
91b3bc2b9c doc: Update documentation generation example in developer-notes.md (Lőrinc)

Pull request description:

  In [the other readmes](https://github.com/bitcoin/bitcoin/blob/6ce50fd9d0ae6850d54bf883e7a7c1bcb6912c5c/src/test/README.md?plain=1#L19) we've provided a default build directory instead, unified the `developer-notes.md` to specify it explicitly.

  In the next commit I've used this default to go over each reference to our binaries and changed their in-source references to the build directory.
  Some of these changes were in example outputs - I haven't validated that the outputs are still the same.
  I haven't modified the build folders in the devtools.

ACKs for top commit:
  maflcko:
    review ACK 6a68343ffb
  pablomartin4btc:
    ACK 6a68343ffb
  fanquake:
    ACK 6a68343ffb - we still need to followup with other scripts/devtools, and likely unify what we are doing in some way, but this is an improvement.

Tree-SHA512: 905d9c68cafe1e405e98d6aa089d7a36a34c9e03403df5c67ac2c9a98cfa54a0305b647cb92247dcb9f49e9b509a8ba88367392b95618c67059684c67b6c36fb

src/qt/README.md

@@ -11,7 +11,7 @@ When following your systems build instructions, make sure to install the `Qt` de
 To run:
 
 ```sh
-./src/qt/bitcoin-qt
+./build/src/qt/bitcoin-qt
 ```
 
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