Merge bitcoin/bitcoin#32417: doc: Explain that .gitignore is not for IDE-specific excludes

fada115cbe doc: Explain that .gitignore is not for IDE-specific excludes (MarcoFalke)

Pull request description:

  This comes up frequently, so document it.

  Examples:

  * https://github.com/bitcoin/bitcoin/pull/27275
  * https://github.com/bitcoin/bitcoin/pull/21894
  * https://github.com/bitcoin/bitcoin/pull/32392
  * https://github.com/bitcoin/bitcoin/pull/17868

  ...

ACKs for top commit:
  l0rinc:
    ACK fada115cbe
  hebasto:
    ACK fada115cbe.
  janb84:
    ACK [fada115](https://github.com/bitcoin/bitcoin/commit/fada115cbeaa8c0ca3d19178499079b66ee5f499)

Tree-SHA512: 6fd1e48de54b40118a49ef61ad157fea278bba568286a022559a4815e53f26650db98a779a1ea2b3085237ded5cbee20d4c2dd1ef51e6a2d3d6085d7063f5309

.gitignore

@@ -1,3 +1,8 @@
+# Patterns that are specific to a text editor, IDE, operating system, or user
+# environment are not added here. They should be added to your local gitignore
+# file instead:
+# https://docs.github.com/en/get-started/git-basics/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer
+
 # Build subdirectories.
 /*build*
 !/build-aux