Clean up `protocol/system-config.md`

[joshklop]

For the config update enum, we should specify the number as well as the canonical name (e.g., MIN_BASE_FEE and 6, not just MIN_BASE_FEE).

For config updates added by hard forks, we should consider linking to fork-specific specs to avoid duplication and spec bloat.

For context and motivation: #767 (comment)

[joshklop]

Suggestion from @sebastianst #767 (comment)