Small improvements to documentation

[jlaehne]

Description of the change

Fix some typos and add image illustrating hyperspectral data to introduction. Fixed some issues with links in the documentation. Adapted the documentation folder structure to other projects in the hyperspy ecosystem (get rid of extra source folder and rename build to _build)

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] update user guide (if appropriate),
• [x] ready for review.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (cf48f32) to head (15566e3). Report is 46 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #193 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 12 12 Lines 554 554 ========================================= Hits 554 554 ```

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[jlaehne]

The change in the folder structure has to be also adapted on ReadTheDocs. Can you add me as maintainer of the LumiSpy project on ReadTheDocs @jordiferrero ?