Update Grid and Guides

Grum 999requested to merge
grum/docs-krita-org:grum999/grid_and_guides into master

Documentation updated to take in account changes made on grid & guides docker implemented with MR 2090 Mostly for users documentation, Isometric (legacy) and Isometric grids

Also reviewed and added screenshots

May be a review for grammar and phrasing could be a good thing 😄

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