Add Word Per Minute calculation feature (#8054)

* Add Word Per Minute calculation feature

* Fix copyright info

* Remove header from quantum.c, setup overloadable keycode inclusion for WPM, update docs

* Simplify logic for keycode filtering

* Adding link from summary to wpm_feature info

* Update docs/feature_wpm.md

Typo in function prototype example in docs

Co-Authored-By: James Young <18669334+noroadsleft@users.noreply.github.com>

* Add WPM transport via i2c

Co-authored-by: James Young <18669334+noroadsleft@users.noreply.github.com>

docs/_summary.md

@@ -80,6 +80,7 @@
     * [Terminal](feature_terminal.md)
     * [Unicode](feature_unicode.md)
     * [Userspace](feature_userspace.md)
+    * [WPM Calculation](feature_wpm.md)
 
   * Hardware Features
     * Displays

docs/feature_wpm.md

@@ -0,0 +1,25 @@
+# Word Per Minute (WPM) Calculcation
+
+The WPM feature uses time between keystrokes to compute a rolling average words
+per minute rate and makes this available for various uses.
+
+Enable the WPM system by adding this to your `rules.mk`:
+
+    WPM_ENABLE = yes
+
+For split keyboards using soft serial, the computed WPM
+score will be available on the master AND slave half.
+
+## Public Functions
+
+`uint8_t get_current_wpm(void);`
+This function returns the current WPM as an unsigned integer.
+
+
+## Customized keys for WPM calc
+
+By default, the WPM score only includes letters, numbers, space and some
+punctuation.  If you want to change the set of characters considered as part of
+the WPM calculation, you can implement `wpm_keycode_user(uint16_t keycode)`
+and return true for any characters you would like included in the calculation,
+or false to not count that particular keycode.