Elevation controller: Add option to set a fixed elevation slice width (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#56846
Author: @nyalldawson
QGIS version: 3.38

**Elevation controller: Add option to set a fixed elevation slice width **

PR Description:

This adds a new option to the configure menu for the elevation controller, for setting a fixed elevation slice width. It can be
used when a specific elevation range width is desired, or when the range should always be zero width.

Commits tagged with [need-docs] or [FEATURE]

"[feature] Add option to set a fixed elevation slice width\n\nThis adds a new option to the configure menu for the elevation\ncontroller, for setting a fixed elevation slice width. It can be\nused when a specific elevation range width is desired, or when\nthe range should always be zero width."