[docs] Fix Doxygen warnings, add CI docs lint job (#3639)

The CI docs lint build is configured to fail on Doxygen warnings.

wpimath/src/main/native/include/frc/fmt/Eigen.h

@@ -8,10 +8,25 @@
 
 #include "Eigen/Core"
 
+/**
+ * Formatter for Eigen::Matrix<>.
+ *
+ * @tparam Rows Number of rows.
+ * @tparam Cols Number of columns.
+ * @tparam Args Defaulted template arguments to Eigen::Matrix<>.
+ */
 template <int Rows, int Cols, int... Args>
 struct fmt::formatter<Eigen::Matrix<double, Rows, Cols, Args...>> {
+  /**
+   * Storage for format specifier.
+   */
   char presentation = 'f';
 
+  /**
+   * Format string parser.
+   *
+   * @param ctx Format string context.
+   */
   constexpr auto parse(fmt::format_parse_context& ctx) {
     auto it = ctx.begin(), end = ctx.end();
     if (it != end && (*it == 'f' || *it == 'e')) {
@@ -25,6 +40,13 @@ struct fmt::formatter<Eigen::Matrix<double, Rows, Cols, Args...>> {
     return it;
   }
 
+  /**
+   * Writes out a formatted matrix.
+   *
+   * @tparam FormatContext Format string context type.
+   * @param mat Matrix to format.
+   * @param ctx Format string context.
+   */
   template <typename FormatContext>
   auto format(const Eigen::Matrix<double, Rows, Cols, Args...>& mat,
               FormatContext& ctx) {

wpimath/src/main/native/include/frc/fmt/Units.h

@@ -8,9 +8,28 @@
 
 #include "units/base.h"
 
+/**
+ * Formatter for unit types.
+ *
+ * @tparam Units Unit tag for which type of units the `unit_t` represents (e.g.
+ *               meters).
+ * @tparam T Underlying type of the storage. Defaults to double.
+ * @tparam NonLinearScale Optional scale class for the units. Defaults to linear
+ *                        (i.e. does not scale the unit value). Examples of
+ *                        non-linear scales could be logarithmic, decibel, or
+ *                        richter scales. Non-linear scales must adhere to the
+ *                        non-linear-scale concept.
+ */
 template <class Units, typename T, template <typename> class NonLinearScale>
 struct fmt::formatter<units::unit_t<Units, T, NonLinearScale>>
     : fmt::formatter<double> {
+  /**
+   * Writes out a formatted unit.
+   *
+   * @tparam FormatContext Format string context type.
+   * @param obj Unit instance.
+   * @param ctx Format string context.
+   */
   template <typename FormatContext>
   auto format(const units::unit_t<Units, T, NonLinearScale>& obj,
               FormatContext& ctx) {