Transliterate a name that ASCII cannot carry, instead of dropping letters #9

slugify (cmd/exposer/index.go) normalises with NFKD and then keeps ASCII
only, mirroring Python's .encode("ascii", "ignore"). Characters that have no
ASCII decomposition are dropped rather than transliterated:

Two separate defects:

1. A name in a non-Latin script has no slug at all. Since Refuse an album, tag or place that has no address of its own #7 that fails the
   build naming the album, tag or place, which is honest but leaves exposer
   unusable for a library whose albums are named in Japanese, Russian, Greek or
   Hebrew. The error message can only suggest renaming the album.
2. A ligature loses a letter silently. Œuvre becomes uvre, which no one
   would choose as an address, and nothing reports it.

Options, roughly in order of cost:

• Transliterate the Latin supplement by hand: ä → ae, ö → oe, ü →
  ue, ß → ss, œ → oe, æ → ae, ø → oe, å → aa. Small, no
  dependency, and the German case is the one this library actually meets.
• A general transliteration table for other scripts. Much larger, and the
  romanisation of a CJK name is not something a table can settle correctly
  (東京 is tokyo in Hepburn, toukyou in Kunrei with long vowels marked).
• Let the library decide: a slug: field in _data/albums/<slug>.yaml,
  _data/tags/ and _data/locations/, so the photographer picks the address
  and exposer never guesses. This also answers the CJK case without a table,
  and it is probably the right answer for an album, which already has a file.

Whichever is chosen, the URLs are a promise: changing how an existing name
slugs moves pages that are already published and already linked. So
transliterating the Latin supplement is a breaking change for any site with an
umlaut in an album name, and needs either a major version or a documented
compatibility switch. Adding an explicit slug: field is additive and breaks
nothing.

Spec: this needs a requirement before it has an implementation. R-3, R-19 and
R-10 all name a slug without saying what one is; the rule belongs in
docs/spec.md §11 beside Q7's photo id.

Related: #7, which made the empty slug a hard failure rather than a broken site.