Collect docs for C++ compilers (GCC, Clang, MSVC)

[henry0816191]

Summary

Scrape and collect documentation for all C++ compilers and toolchains listed in the investigation. Doc sources need to be identified.

Tools & known sources

• GCC: TBD (identify official docs)
• Clang: github
• MSVC: TBD (identify official docs)

Acceptance criteria

• Doc sources identified for GCC, Clang, and MSVC.
• Documentation scraped/collected for each compiler.
• Output format and storage location agreed and documented.

[jonathanMLDev]

I've already collected and upserted the data of GCC and MSVC.
for clang, I will clone the github repository, and then preprocess and upsert all *.rst files.