chore: add .build to .gitignore (XRPLF#4722)

Currently, the `BUILD.md` instructions suggest using `.build` as the
build directory, so this change helps to reduce confusion.

An alternative would be to instruct developers to add `/.build/` to
`.git/info/exclude` or to user-level `.gitignore` (although the latter
is very intrusive). However, it is being added here because it is a good
practice to have a sensible default that's consistent with the build
instructions.

.gitignore

@@ -109,3 +109,6 @@ pkg
 CMakeUserPresets.json
 bld.rippled/
 .vscode
+
+# Suggested in-tree build directory
+/.build/