Updates to main window and left sidebar chapters

I am back to working my way through the entire digiKam english-language manual in effort to improve readability.

I moved the Left Sidebar section out of the Main Window chapter, making the Left Sidebar a separate chapter. This change is consistent with the fact that the Right Sidebar is a chapter.

I replaced the old figure illustrating the window layout, with a new figure that shows all of the areas discussed in the text, and includes labels making it easy to see the different areas. I made three versions of this figure. One labels each area with English language text, the second uses English text plus numbers, and the third uses only numbers. I ended up including the English plus numbers in this merge request. I thought this would be easier to produce a table for translated versions. Please let me know if you want a different choice made.

digiKam suggestion: Change the top entry in the Albums list from 'Albums' to 'Collections'. This would then be an indication that the second level in this list are indeed collections, not albums. Albums are then the subfolders within a collection.

Many minor grammar updates.