Lots of Doxygen tweaks and fixes (#212)

* Add documentation for gpio_irq_callback_t and rtc_callback_t (fixes #175)
 * Hook up pico_binary_info and pico_bootsel_via_double_reset to the Doxygen index
 * Add link to new Raspberry Pi Pico FAQ
 * Consistently add parentheses for \sa or \see links to function-names
 * Use consistent capitalisation for 'GitHub'
 * Small typos and markup errors

src/common/pico_base/include/pico/types.h

@@ -21,8 +21,8 @@ typedef unsigned int uint;
     The type is used instead of a raw uint64_t to prevent accidentally passing relative times or times in the wrong
     time units where an absolute time is required. It is equivalent to uint64_t in release builds.
 
-    \see to_us_since_boot
-    \see update_us_since_boot
+    \see to_us_since_boot()
+    \see update_us_since_boot()
     \ingroup timestamp
 */
 #ifndef NDEBUG

src/common/pico_binary_info/include/pico/binary_info.h

@@ -7,7 +7,9 @@
 #ifndef _PICO_BINARY_INFO_H
 #define _PICO_BINARY_INFO_H
 
-/**
+/** \file binary_info.h
+ *  \defgroup pico_binary_info pico_binary_info
+ *
  * Binary info is intended for embedding machine readable information with the binary in FLASH.
  *
  * Example uses include:
@@ -26,4 +28,4 @@
 #if !PICO_NO_BINARY_INFO
 #include "pico/binary_info/code.h"
 #endif
-#endif
+#endif

src/common/pico_binary_info/include/pico/binary_info/code.h

@@ -25,12 +25,14 @@
 #endif
 /**
  * Declare some binary information that will be included if the contain source file/line is compiled into the binary
+ * \ingroup pico_binary_info
  */
 #define bi_decl(_decl) __bi_mark_enclosure _decl; __bi_decl(__bi_ptr_lineno_var_name, &__bi_lineno_var_name.core, ".binary_info.keep.", __used);
 /**
  * Declare some binary information that will be included if the function containing the decl is linked into the binary.
  * The SDK uses --gc-sections, so functions that are never called will be removed by the linker, and any associated
  * binary information declared this way will also be stripped
+ * \ingroup pico_binary_info
  */
 #define bi_decl_if_func_used(_decl) ({__bi_mark_enclosure _decl; __bi_decl(__bi_ptr_lineno_var_name, &__bi_lineno_var_name.core, ".binary_info.", ); *(volatile uint8_t *)&__bi_ptr_lineno_var_name;});
 
@@ -136,4 +138,4 @@ static const struct _binary_info_named_group __bi_lineno_var_name = { \
 #define bi_3pins_with_names(p0, name0, p1, name1, p2, name2)  bi_pin_mask_with_names((1u << (p0)) | (1u << (p1)) | (1u << (p2)), name0 "|" name1 "|" name2)
 #define bi_4pins_with_names(p0, name0, p1, name1, p2, name2, p3, name3)  bi_pin_mask_with_names((1u << (p0)) | (1u << (p1)) | (1u << (p2)) | (1u << (p3)), name0 "|" name1 "|" name2 "|" name3)
 
-#endif
+#endif

src/common/pico_time/include/pico/time.h

@@ -77,7 +77,7 @@ static inline uint32_t us_to_ms(uint64_t us) {
  * \brief Convert a timestamp into a number of milliseconds since boot.
  * \param t an absolute_time_t value to convert
  * \return the number of microseconds since boot represented by t
- * \sa to_us_since_boot
+ * \sa to_us_since_boot()
  */
 static inline uint32_t to_ms_since_boot(absolute_time_t t) {
     uint64_t us = to_us_since_boot(t);
@@ -171,7 +171,7 @@ extern const absolute_time_t nil_time;
  * \ingroup timestamp
  *  \param t the timestamp
  *  \return true if the timestamp is nil
- *  \sa nil_time()
+ *  \sa nil_time
  */
 static inline bool is_nil_time(absolute_time_t t) {
     return !to_us_since_boot(t);
@@ -230,7 +230,7 @@ void sleep_ms(uint32_t ms);
 /*! \brief Helper method for blocking on a timeout
  * \ingroup sleep
  *
- * This method will return in response to a an event (as per __wfe) or
+ * This method will return in response to an event (as per __wfe) or
  * when the target time is reached, or at any point before.
  *
  * This method can be used to implement a lower power polling loop waiting on