Fix ASR documentation

[Smit-create]

The ASR documentation still uses the Fortran reference which should be removed and made generalized since it is a standalone library. The file that needs to be updated is: https://github.com/lcompilers/lpython/blob/main/src/libasr/ASR.asdl

[certik]

Yes. This is on @HaoZeke's TODO list as part of GSoD.

[suhanigarg29]

Can this be assigned to me?

[Smit-create]

Please free to work on this and raise a PR.