Postgres/Redis docs: list supported regions per cloud

[lionello]

Some clouds do not support all managed services in all regions, eg. Azure does not support postgres in westus2.

Lets list supported regions per cloud in the docs for the managed (x-defang-…) services.

[defangdevs]

Plan:

Researched region availability for the managed services behind x-defang-postgres / x-defang-redis on each cloud:

• AWS — Amazon RDS and Amazon ElastiCache are included in every AWS Region launch (per AWS's own regional-services page). No per-region gap to document.
• GCP — Cloud SQL for PostgreSQL and Memorystore for Redis are both available in essentially all GCP regions (Google's own docs: "Cloud SQL is available in all Google Cloud regions"; Memorystore lists 40+ regions with no gaps relative to other GCP services). No per-region gap to document.
• Azure — this is the real gap the issue is about. Azure Database for PostgreSQL Flexible Server and Azure Managed Redis do not ship to every Azure region at once, and Microsoft's own docs point to a live "Products available by region" page rather than a static list (Azure Managed Redis: "continually expanding into new regions... to check availability by region, see Products available by region"). On top of that, Defang auto-selects a specific VM SKU per tier (provider/defangazure/azure/postgres_sku.go, redis.go in pulumi-defang) — even in a region where the service is generally available, that specific SKU can hit regional capacity limits (this matches real user reports of Postgres Flexible Server provisioning failures in specific regions/subscription types).

Since Azure's own availability list changes over time, I'm not going to hardcode a static per-region table (it would go stale immediately) — instead:

1. docs/providers/azure.md — expand the existing "Location" section with an explicit callout that Postgres/Redis availability varies by region, link to Azure's live "Products available by region" page, and note that SKU capacity can vary within a supported region too.
2. docs/providers/aws.md / docs/providers/gcp.md — add a one-line note under Managed Storage confirming RDS/ElastiCache and Cloud SQL/Memorystore are available in (essentially) all regions, so users don't have to wonder.
3. docs/concepts/managed-storage/managed-postgres.mdx and managed-redis.md — add a short "Region Availability" pointer in the Current Support section linking to each provider's section instead of duplicating.

Will open a PR referencing this issue; not self-merging (defang-docs isn't one of the repos I self-merge in).

[defangdevs]

Opened #424 with the region-availability docs. Not self-merging — defang-docs isn't a repo I merge in on my own, so this is ready for review.