document linux file descriptor limits

what changed

added a linux file descriptor limits note to the manual. it explains why high-cpu systems can hit Too many open files, shows how to raise the limit before launching the profiled program, and covers the sudo case.

why

tracy can open a file descriptor per cpu while collecting tracepoint data. the default soft limit can be too low on machines with many cpus, which can also stop the profiled application from opening files.

checks

• git diff --check passes.
• updated both manual/tracy.tex and the checked-in manual/tracy.md rendering.
• the local machine does not have a latex or markdown renderer installed, so i did not render the manual locally.

fixes #512