docs: some little documentation tweaks for the 0.4.0 release

[sbillinge]

Problem

no problem, just a few cleaning suggestions.

Proposed solution

• remove the note that the gui requires Python <=3.12. This is not true if the installation instructions install gooey using pip
• Maybe have a getting started section that just points to the example. Just easier to navigate.
• Let's change the filenames of the example files to have dashes instead of underscores (new group standard). Also, let's change that the corrected file is called <name>-corrected.chi and not <name>_corrected.chi (this would be a small code change). Also, I dislike this name because it doesn't capture what was corrected. Let's think about it but maybe <name>-abs-corrected-<mur-value>.chi
• The section that begins To save the correction file, specify the -c or --output-correction flag,... there is no text about why one might want to do that, and that in general it is not needed.
• The reviewers of the paper wanted us to move everywhere to MuR from MuD. I don't love that, but it may be ambiguous if it is sometimes mur and sometimes mud. Shall we change everywhere to mur?
• z-scan is introduced everywhere as if everyone knows what that is, but noone does. It is well described in the paper so we don't really have to reproduce that here. But a sentence about what it is, and a mention to read the paper, when it is first introduced both at the top of the instructions and also in the example section would be good. A link to the diffpy.utils docs is present (good) but also mention the paper.
• instead of <name>_cve.chi for the correction file, again, how about making it less cryptic, something like <name>-cve-correction-<mur-value>.chi. We could call it "abs-correction" but then the difference between the corrected data and the correction itself would be hard to differentiate because the difference would just be abs-corrected vs abs-correction. Maybe calling it mur-correction instead of cve-correction though?