Drop link to Python benchmarks, and copyedits

[chalin]

• This is a proposed fix for Python: performance benchmarks link results in 404 #2354
• Drops the link to the benchmarks page, which is currently offline. For context, see Python: performance benchmarks link results in 404 #2354 (comment).
• Fixes code-blocks for commands: changes the language and drops prompts since the blocks contain no command output
• Other minor copyedits

Preview: https://deploy-preview-2598--opentelemetry.netlify.app/docs/instrumentation/python/