Add scale up and scale down docs

[nomirosa]

There should be clearer documentation covering how to scale clusters up and down (and the implications the latter has in terms of manually setting the desired number of replicas).

This should be available in some form at the level of our current Guide, i.e. not too technically detailed but descriptive of the steps in question.

checklist:

• There is a page in the Guide that tells you how to scale clusters up
• There is a page in the Guide that tells you how to scale clusters down. This page tells you how to manually set the desired number of replicas such that the sum of replicas +1 (for the original) does not exceed the target nodes.
• Other pages ("Scaling Intro", etc.) in the Guide have been updated (and restructured where necessary) so that they fit in with and reference the scale up and scale down docs
• Where appropriate, pages in the Reference and other docs projects link to the scale up and scale down docs
• https://github.com/crate/crate-howtos/issues/175

[nomirosa]

see crate/crate-howtos#180

[nomirosa]

see crate/crate-howtos#159

[nomirosa]

from elsewhere:

• Best Practice
• Needed changes in configuration
• Consequences/tradeoffs/pitfalls

[matthijskrul]

Include https://github.com/crate/new-tech-content/issues/46