docs: Format quoted command references as code in env vars table

Description

The environment variables table on docs/source/_index.md rendered command and config-key references such as 'glab config set browser mybrowser' as single-quoted plain text instead of code.

Rather than editing the help text content in internal/commands/root.go, this change updates the doc generator (cmd/gen-docs/docs.go) to convert single-quoted segments in environment variable descriptions into backtick-wrapped code. A new codeWrapQuoted helper applies this before URL wrapping, so all quoted references in the env vars table render with code formatting automatically.

The generated docs/source/_index.md is updated to match the new generator output.