Add referential actions' links for warnings handling Postgres and MySQL special cases

[jkomyno]

• /d/postgres-set-null is used in the warning message added for https://github.com/prisma/prisma-private/issues/198
• /d/mysql-set-default is used in the warning message added for Add SetDefault validation error with warnings when provider = "mysql" and relationMode = "foreignKeys" | default prisma#16259
• /d/relation-mode-prisma-indexes is used in the warning message added for Recommend adding indices to foreign keys when referentialIntegrity = "prisma" language-tools#992

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Name	Status	Preview	Updated
pris-ly	✅ Ready (Inspect)	Visit Preview	Nov 25, 2022 at 4:03PM (UTC)

[Jolg42]

Note: Alberto is waiting for his docs PR to be ready before merging this

[jkomyno]

Suggested change

	{
	"source": "/d/postgres-set-null",
	"destination": "https://www.prisma.io/docs/concepts/components/prisma-schema/relations/referential-actions#types-of-referential-actions"
	},
	{
	"source": "/d/mysql-set-default",
	"destination": "https://www.prisma.io/docs/concepts/components/prisma-schema/relations/referential-actions#types-of-referential-actions"
	},
	{
	"source": "/d/postgres-set-null",
	"destination": "https://www.prisma.io/docs/concepts/components/prisma-schema/relations/referential-actions#special-cases-for-referential-actions"
	},
	{
	"source": "/d/mysql-set-default",
	"destination": "https://www.prisma.io/docs/concepts/components/prisma-schema/relations/referential-actions#special-cases-for-referential-actions"
	},

[jkomyno]

This mirrors the preview link mentioned in: prisma/docs#4132 (comment)

[janpio]

Let's make sure the links at least exist (and can not be forgotten), and then follow up via another PR.

[janpio]

And #80