Add signal tools page to documentation

[jlaehne]

Requirements

Split signal tools page out of the `utilities page in the user guide. Add some important HyperSpy functions. Some additional small improvements to the documentation.

Closes #171.

Progress of the PR

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

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (e35e472) compared to base (8367838). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #173 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 548 503 -45 ========================================= - Hits 548 503 -45 ``` | [Impacted Files](https://codecov.io/gh/LumiSpy/lumispy/pull/173?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy) | Coverage Δ | | |---|---|---| | [lumispy/signals/luminescence\_spectrum.py](https://codecov.io/gh/LumiSpy/lumispy/pull/173?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS9zaWduYWxzL2x1bWluZXNjZW5jZV9zcGVjdHJ1bS5weQ==) | `100.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy)

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[jlaehne]

I like the documentation like this much more as the utils section was a bit confusing.

I agree, I'll try to extend and improve it a bit further the next days.