ftxdiff: add page (#9177)

* Create ftxdiff

from page request: macOS Font Tools aka. Apple Font Tool Suite #8478 
Note: I'm a new dev, but experienced writer. This is my first docs PR. Feedback appreciated.

* Fix typos in ftxdiff.md

Fix typos including "Ending command descriptions with a period" "Example descriptions should end in a colon with no trailing characters"

* Update pages/osx/ftxdiff.md

* Update pages/osx/ftxdiff.md

add {{ FileName }} around file names

* Update pages/osx/ftxdiff.md

* Update pages/osx/ftxdiff.md

* Update pages/osx/ftxdiff.md

delete extra space

* Update pages/osx/ftxdiff.md

Add space between : and < in url

* Update pages/osx/ftxdiff.md

delete extra space on line 3

* Update pages/osx/ftxdiff.md

* Apply suggestions from code review as submitter failed to address them

pages/osx/ftxdiff.md

@@ -0,0 +1,16 @@
+# ftxdiff
+
+> Compare differences between two fonts.
+> More information: <https://developer.apple.com/fonts>.
+
+- Output differences to a specific text file:
+
+`ftxdiff --output {{path/to/fontdiff.txt}} {{path/to/font1.ttc}} {{path/to/font2.ttc}}`
+
+- Include glyph names in output:
+
+`ftxdiff --include-glyph-names`
+
+- Include unicode names in output:
+
+`ftxdiff --include-unicode-names`