Add developer guideline to use east const. (#14836)

This PR documents the libcudf preference for "east const." Follow-up from #13491, #13492, #13493, #13494.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Nghia Truong (https://github.com/ttnghia)
  - Yunsong Wang (https://github.com/PointKernel)

URL: #14836

cpp/doxygen/developer_guide/DEVELOPER_GUIDE.md

@@ -129,6 +129,10 @@ and we try to follow his rules: "No raw loops. No raw pointers. No raw synchroni
 
 Additional style guidelines for libcudf code include:
 
+ * Prefer "east const", placing `const` after the type. This is not
+   automatically enforced by `clang-format` because the option
+   `QualifierAlignment: Right` has been observed to produce false negatives and
+   false positives.
  * [NL.11: Make Literals
    Readable](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#nl11-make-literals-readable):
    Decimal values should use integer separators every thousands place, like