TLS for name owners: document 520-byte name-value limit and how to conserve space

[mstrofnone]

Adds a new ## Conserving Blockchain Space section to docs/name-owners/tls/index.md, inserted between the existing Cipher Suites and Using Your Own Tooling sections (alongside the other short operational-tips sections at the bottom of the page).

The 520-byte name-value cap is currently mentioned in the FAQ but not on the TLS page, even though it directly affects how name owners structure their tls records, especially when also juggling ip, map, ns, import, etc. This section:

• States the 520-byte cap with a link to the existing FAQ entry.
• Quantifies the on-chain cost of Compressed-mode vs Hashed-mode TLSA records (~80-120 bytes vs ~50 bytes), giving readers a concrete reason to prefer Hashed mode on top of the security tradeoffs already in Concepts.
• Points readers at [Delegated Alteration]({{ "/docs/name-owners/delegated-alteration" | relative_url }}) as the canonical way to factor common subtrees into a paired dd/ name when space is tight, noting that keeping the tls field in the d/ name is also a useful anti-MITM hardening.

Purely additive. No existing content is changed.