docs: raise alternative origins limit to 100 (#349)

## Summary

- Internet Identity raised the cap on entries in
`/.well-known/ii-alternative-origins` from 10 to 100
(dfinity/internet-identity#4261, merged). This updates the docs to
match.
- `docs/guides/authentication/internet-identity.mdx`: the alternative
origins step said "A maximum of 10 alternative origins can be listed";
now 100.
- Supersedes dfinity/portal#6248, which made the equivalent change
before that repo was retired.

## Scope

The limit also appears three times in
`docs/references/internet-identity-spec.md` (requirements list, the JSON
schema `maxItems`, and the "prevent misuse" note). That file is
generated by `scripts/sync-ii-spec.mjs` from the pinned
`.sources/internetidentity` submodule and carries a "do not edit
directly" marker, so it is deliberately untouched here. The upstream
`docs/ii-spec.mdx` already has the new value, so the page picks it up
when that submodule is bumped and the sync is rerun. Happy to do that in
a separate PR if you would like it in the same release.

No other page needed changing: the remaining "at most 10" hits in
`docs/references/execution-errors.md` and
`docs/references/ic-interface-spec/management-canister.md` are about
canister controllers.

## Verification

- `npm run build` passes (210 pages).
- Grepped `docs/` for the limit in every phrasing; only the generated
spec page still says 10, for the reason above.

Co-authored-by: Claude <noreply@anthropic.com>

Author: aterga

docs/guides/authentication/internet-identity.mdx

@@ -569,7 +569,7 @@ To keep principals consistent across your own custom domains, configure **altern
    }
    ```
 
-   A maximum of 10 alternative origins can be listed. No trailing slashes or paths.
+   A maximum of 100 alternative origins can be listed. No trailing slashes or paths.
 
 2. **Configure the asset canister** to serve the `.well-known` directory. Add an `.ic-assets.json5` in your frontend source: