Populate Using ExecuTorch with C++ doc page

[GregoryComer]

📚 The doc issue

Note: This issue pertains to the executorch-just-works branch. It is intended to merge back into master within the next 1-2 weeks.

Populate the "Using ExecuTorch with C++" doc page under using-executorch-cpp.md. Describe high-level and low-level runtime APIs, and when to use each. Cover CMake build integration and relevant targets.

General documentation guidelines that I'm hoping to follow:

• Leverage brief in-line code samples, where possible
• Keep things concise
• Avoid first person (I/we)
• I'm generally trying to follow https://developers.google.com/style/highlights

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw