docs: add a PvP spell cooldowns section

[Kheartz]

Adds a Spell cooldowns section to the PvP Hits and defeat guide, plus a cooldowns row in the rules table on Rules and relations.

The section covers:

• when a player counts as in a PvP fight: a hostile player within 50 m, or a hit within flagCombatLockSeconds;
• that spells cast in a fight wait their full cooldown;
• a table of in-fight and out-of-fight times for the common combat spells;
• the cooldowns rule: "full" (the default) or "fast".

The flagCombatLockSeconds row now also says that it sets how long a hit keeps a player in a fight.

Depends on the mod change. The cooldowns rule comes from the mod's pvp_cooldowns branch, which isn't merged yet. Until that change is merged and the testing contract is republished, docs:check fails on the new sample:

guides/PvP/hits-and-defeat.md:104:17 - TS2353 (server): ... 'cooldowns' does not exist in type 'Partial<PvpRules>'.

Merge this after the mod change.

Tested locally:

• docs:check: all 170 samples type-check against a contract built from the mod branch;
• pnpm build and docs:links: every link and anchor on 48 pages resolves;
• viewed the built page.