Disambiguate colliding page titles (Overview x3, Bridging, Escrow, Encrypted Payment Comments)

Follow-up from #2237.

Four titles collide across nine pages. Titles are faithful to frontmatter (zero mismatches across all entries) — the pages themselves are just named ambiguously.

This matters most for `llms.txt`, where an assistant picking a page to fetch sees only the title:

- **`Overview`** ×3
- `infra-partners/notices/overview`
- `legacy/overview`
- `legacy/validator/key-management/summary`
- **`Bridging`** ×2 — `home/bridged-tokens/bridges`, `tooling/bridges/bridges`
- **`Escrow`** ×2 — `home/protocol/escrow`, `legacy/protocol/transaction/escrow`
- **`Encrypted Payment Comments`** ×2 — `home/protocol/transactions/tx-comment-encryption`, `legacy/protocol/transaction/tx-comment-encryption`

## Fix

Disambiguate `title:` in frontmatter. `sidebarTitle:` can keep the short form, so the sidebar is unchanged — e.g. `title: "Network Notices Overview"` with `sidebarTitle: "Overview"`.

The `home/` vs `legacy/` pairs mostly need the legacy one marked as such, since the collision is L1-vs-current rather than two genuinely different topics.

Note: largely mitigated for free if #2239 lands, since descriptions disambiguate too. Worth doing anyway for the sidebar and search.

Open the nine listed pages and inspect their title and sidebarTitle frontmatter, starting with the three Overview paths and the home/legacy pairs. Keep short sidebar labels where specified, but make each full title identify its topic or legacy status. Done means the four collision groups no longer share ambiguous full titles while the sidebar remains unchanged.

Written by the indexing model from the issue text.