Keep slashless redirects on HTTPS

## Problem

Production requests for canonical docs routes without a trailing slash return an absolute `http://` redirect even when the original request uses HTTPS.

Examples:

```console
$ curl -I https://docs.miden.xyz/next/builder/migration/asset-vault-faucet
HTTP/2 301
location: http://docs.miden.xyz/next/builder/migration/asset-vault-faucet/

$ curl -I https://docs.miden.xyz/0.15/reference/protocol/asset
HTTP/2 301
location: http://docs.miden.xyz/0.15/reference/protocol/asset/
```

The trailing-slash URLs themselves return HTTPS 200, and generated site links now use trailing slashes, so normal navigation works. Directly entered or externally linked slashless URLs still take an unnecessary insecure intermediate hop.

## Expected behavior

Slashless HTTPS requests should either:

- redirect directly to the trailing-slash HTTPS URL, or
- serve the canonical route without an HTTP downgrade.

## Scope

This appears to be a hosting/proxy canonicalization issue rather than a Docusaurus content redirect. Check the GitHub Pages and Cloudflare redirect/proxy configuration.

## Acceptance criteria

- `Location` remains `https://docs.miden.xyz/.../` for slashless HTTPS requests.
- Current, next, and versioned routes behave consistently.
- Existing trailing-slash routes and Docusaurus client redirects continue to work.