Document string schema custom attributes

This commit adds changes to document string schema custom attributes. Community at truecharts raised a concern where they are not sure how the regex works for valid_char string attribute, so the changes introduced in this commit address the concerns and while we are here document other custom string options as well.

Upgraded catalog item(s)

        This commit upgrades chia, minio catalog item(s).

Upgraded catalog item(s)

        This commit upgrades chia, minio catalog item(s).

Upgraded catalog item(s)

        This commit upgrades chia, minio, nextcloud catalog item(s).

Use custom image for chia

Update test train catalog items

Upgraded catalog item(s)

Document scaling down app on upgrade for snapshot

        This commit upgrades chia, ipfs, minio, plex catalog item(s).

This commit documents change how chart devs can have certain workloads to be scaled down before taking a snapshot of the application's volumes on an upgrade of the application to a newer catalog item version.

Update charts for 21.06

Add metadata to allow automated updates of catalog item(s)