RDoc-3556 better docs for automatic storage extension - #2393

Fold in the content of the closed RDoc-3556 PR (ravendb#2393), re-validated
against hr/test rather than carried over as written.

- Availability: Development and Production tiers, excluding Dev10 and
  PN-type instances (ProductFeaturesHelper.IsFeatureAvailable). Note that
  ineligible products have no feature row at all.
- Enabled by default at 90%, and the feature itself is free: its pricing
  percentage is 0 (ProductRuntimeCostsCalculator.GetFeaturePercentage),
  so only the larger disk is billed.
- How it works: nodes report usage to /api/v2/node/storage/auto-extend and
  the disk moves to the next size the instance type offers, not to the size
  that would merely clear the threshold. The success email carries the old,
  new and maximum size (AutoStorageExtend.html).
- Azure Premium disks also gain the new size's IOPS and throughput when the
  target performance tier is better (ExtendProductStorageVolumeHelper).
- When storage is not extended: the four reasons that send an email
  (instance-type maximum, root-disk limit of 1000/1024/1000 GB on
  AWS/Azure/GCP for root-only layouts, yearly contract, Free tier), at most
  one per product per 24 hours (EmailNotificationPolicy).
- Storage modification rate: 6 h on AWS and 12 h on Azure Premium between
  two storage changes (Constants.StorageChangeLimitExpirationMinutes). This
  gates automatic and manual changes alike and was undocumented, so it is
  also noted on the Scaling page.
- Enabling and disabling: Disable/Enable buttons, gated on ManageFeatures
  (Manager or Admin preset) and an Active product.

Explicit slug on the new Configuration heading so it does not become
configuration-3 among the page's other Configuration sections.

Cross-link from Scaling - Change Storage to the feature, which ravendb#2393 also
added.