Fix typos in the user guide - multiple PRs welcome!

[micaela-matta]

This issue is designed to be incrementally achieved through multiple PRs.

There are a few typos in the user guide, this issue will collect as many as we can find starting with the short list below. Feel free to fix any typos/formatting errors we haven't noticed and add them in the comments!

• units and conversions page, note box: "in the future me might"
• units and conversions page, above the note box: fix LaTeX formatting for rho/rho0 = n/(N_A * M**-1) / rho0 where [n] = 1/Volume, [rho] = mass/Volume
• units and conversions page, MDAnalysis.units.water: fix scientific notation inand molar mass 18.016 g mol**-1
• density analysis page, units and conversions [page]https://docs.mdanalysis.org/2.1.0-dev0/documentation_pages/units.html?highlight=unit%20conversion#conversions) (and probably many other pages): the symbol Å appears many times in different formats (ångströms, Å**-3, Angstroms etc). Depending on the context in the page, keep LaTeX equation style or replace with the standard symbol Å and correct scientific notation.

[lilyminium]

Sorry, all of these typos should be fixed in the core MDAnalysis/mdanalysis repo!