Updating usage and examples.

[ajarifi]

I edited the usage and examples by adding the infinite potential well and hydrogen-like atom in the README.md and docs/src/index.md. Probably, the example is too much for README.md.
I also created a table of symbols and Greek letters, which may be useful for the current version.
Two plots for wave functions are also uploaded.

[ohno]

Sorry for the late response and for throwing away your contribution. I've added some comments so please update PR.

[ohno]

Please see #88. Please do not add contents to README.md. It will be simple like https://github.com/JuliaFewBody/FewBodyHamiltonians.jl.

[ohno]

Please do not commit image files. The following usage is sufficient for use in documentation. Images are generated automatically by GitHub Actions and Documenter.jl; there's no need to include them in your source code.

Antique.jl/docs/src/InfinitePotentialWell.md

Lines 74 to 75 in 19a245c

	f
	```

Antique.jl/docs/src/InfinitePotentialWell.md

Lines 99 to 104 in 19a245c

	f
	save("assets/fig/InfinitePotentialWell.png", f) # hide
	; # hide
	```
	

[ohno]

The comment is same as docs/src/assets/fig/H_wavefunction.png.

[ohno]

This is too much to solve #100. It would be enough if the usage of E, V, and ψ were covered as pointed out in the issue. Either a harmonic oscillator or a hydrogen atom would be enough. Please keep your contribution to the Greek Letters and Symbols table for this version.