docs: PySDM docs improvement

[AgnieszkaZaba]

@emmacware, @pawelmagnu, @Sfonxu, @olastrz

Notes from today's weekly meeting:

• [x] add list of references (same as for PyMPDATA)
• [ ] add equations for sections in PySDM (maybe some general one). We can make separate issue for this or discussion on what equations will describe a section.
• [ ] add tags to a papers to make it easier to navigate.
• [x] Create a list with notebook examples with short explanation what we can find inside - no need to open a notebook or a paper when navigating.
• [x] #1434

Other issues connected to documentation:

• #1427
• #1426
• #1429

[slayoo]

Thank you! Other relevant issues worth mentioning here:

• #1428
• #1203
• #1152

[slayoo]

reference list added in #1483 kudos @pawelmagnu !