Add DNS host label length warning near route/hostname examples

[NHingerl]

Problem

The Deploy Multitenant SaaS Applications guide shows how tenant-specific URLs are constructed (https://<subaccount subdomain><separator><appName>.<domain>), for example:

cf map-route bookshop cfapps.us10.hana.ondemand.com --hostname subscriber1-myOrg-mySpace-bookshop

The segment before the first dot (the hostname) becomes a single DNS host label, which has a hard limit of 63 characters (RFC 1035). When customers use long subdomain names, org/space names, or app names, this limit is easily exceeded, causing deployment failures or broken routes.

Suggested change

Location 1: guides/multitenancy/index.md, near the cf map-route example

After the cf map-route code block, add a warning:

::: warning DNS host label limit
The hostname (everything before the first `.`) must not exceed 63 characters.
If `<subscriberSubdomain>-<saasAppName>` is too long, the route will fail.
Use shorter app names or configure an explicit short route via the `routes` parameter in your `mta.yaml`.
:::

Location 2: same file - near the CDS_MULTITENANCY_APPUI_URL table

After the line showing https://<subaccount subdomain><separator><URL>, add a tip:

::: tip Keep hostnames short
The resulting hostname must stay within the 63-character DNS host label limit.
Choose short values for subdomain, separator, and app URL components.
:::

Location 3 (optional): get-started/get-help.md - MTXS or Cloud Foundry section

A short troubleshooting entry:

#### Why does my tenant URL fail or are truncated? {#hostname-exceeds-63-characters}

DNS host labels are limited to 63 characters. The tenant-specific hostname (`<subdomain><separator><appName>`) can exceed this limit with long names.

**Solution:** Use shorter component names or set an explicit short route using the `routes` parameter in your `mta.yaml`.

Context

We're updating the BTP Admin Guide: Naming and Directory Templates with a general note about this limit. We're also reaching out to the BTP Core docs team about the MTA Routes topic.