Bring documentation on 'Directory Structure' up-to-date

[ashmaroli]

The documentation on 'Directory Structure' contains a few dated information and errors that can confuse a new Jekyll user. This pull fixes those errors.

• Add a note on differing directory structure for sites created with Jekyll v3.2+
• Add a note about _sass directory contents.
• Fix any remaining typos or errors in general.

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Screenshot of the new info panel:

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/cc @jekyll/documentation

[DirtyF]

Thanks for bringing this up @ashmaroli, already had to explain this several times to some 😕 users

I tried to make it easier to grasp:

/cc @dret

[ashmaroli]

Thank you @DirtyF, 😃 it reads better now.. I found few places that needed some minor edits and have made those..

[parkr]

@jekyllbot: merge +site