feat: improve title and description in embeds

[Chew]

Currently, sending a docs link on Discord just always returns RetroAchievement Docs with no description or title or anything.

Now, it will return the full title and description.

A titleTemplate was added, meaning | RetroAchievements Docs is added to titles.

The description pulls from the description: in the front-matter.

[greptile-apps]

Confidence Score: 4/5

Safe to merge; change only affects meta tags injected into page head.

Small, well-scoped change. The only rough edge is that pages missing a description frontmatter will emit an empty twitter:description tag rather than omitting it.

docs/.vitepress/config/shared.ts — the transformHead description guard.

_{Reviews (1): Last reviewed commit: "feat: improve title and description in e..." | Re-trigger Greptile}

[wescopeland]

Thanks!