serialosc docs cleanup

[dndrks]

based on recent conversations with @ryleelyman and @tehn!

this PR addresses a few points in the serialosc docs:

• serial.txt didn't have the correct formatting for grid messages. eg. i changed /prefix/led/set x y s to /prefix/grid/led/set x y s
• the introductory walkthrough in the serialosc landing page required more familiarity than a 'first timer' might be able to grok, so i added some oscsend and oscdump formatting to each of the code examples
• the osc.md serialosc reference was missing <prefix> callouts and argument manifests (eg. iii), which are required for successful communication

@tehn , please lmk if any of this is too heavy handed for your tastes and i can scale back! @ryleelyman, did you run into any other docs stumbling blocks during your seamstress development?

[dndrks]

alright, got all the stuff fluffed (thank you for the super-helpful review!) + added some detail about the preferences file in the serialosc detailed page. please lmk if there's anything that feels wonky!