docs: reword README intro to lead with use case

[peo-machine]

Summary

Sync the README intro paragraph with the new landing-page wording from 3ncr.org/3ncr.org#11. Leads with what 3ncr is for (config values, tokens, app secrets) and the cross-language interop story instead of the old "standard for string encryption / decryption" framing.

Before:

3ncr.org is a standard for string encryption / decryption (algorithms + storage format), originally intended for encrypting tokens in configuration files but usable for any UTF-8 string.

After:

3ncr.org is a tiny encrypted-string format for config values, tokens, and app secrets — encrypt a UTF-8 string in one language, store it as a recognizable value, and decrypt it in another.

Test plan

• Diff is README-only, three lines.