RSDK-3836 - update documentation

[purplenicole730]

The docs have been updated to be more consistent with the existing examples in the Python SDK as well as the official Viam docs.
The changes include:

• adding a simpler Sensor example and adding the Arm example as an additional example at the bottom of the Modular Resources section
• suggesting users to go to modular resources before trying custom remotes
• updating the module example's README to mention the new Arm modular component
• in the README, point readers to go to the Python docs if they want to see a more in-depth explanation on how to create a modular component from an existing resource
• basic wording fixes

NOTE: Some of the wording is based off of the actual Viam docs for consistency.
The custom resource Gizmo and Summation module examples were not added to the docs on purpose. It seemed too repetitive without a lot of payoff, as it would be wordy for something that can be more easily shown in GitHub the way it is now. The docs have been updated to point a user to the example, and the README has also been updated to point a reader to the docs for existing resources.

[andf-viam]

Awesome! I've noted some small optional language suggestions for your consideration. Thanks!

[benjirewis]

LGTM! Thanks 🧑‍🔧

[cheukt]

nit: pr name should be in form of RSDK-XXXX blah, makes things easier to track and also makes our pr titles more consistent