citation to FriCAS

[Qian Yun]

There's more and more papers mentioning FriCAS recently.
(doing full text search in arxiv.org)

Searching on google scholar reveals:

3 citations for "FriCAS—An advanced computer algebra system",
2 citations for "FriCAS 1.3. 10—an advanced computer algebra system",
2 citations for "FriCAS—an advanced computer algebra system (2024)"

We should try to unify them.

I suggest to remove package version in
src/doc/sphinx/source/citation.rst, and use a fixed year.

That seems to be the common practice for software citation.

Opinions?

- Qian

[Qian Yun]

Or the citation should point to the reference book?

- Qian

[Qian Yun]

Just found out that github supports the "CITATION.cff" file.

And we can apply for a DOI at https://zenodo.org/ .

We should consider that.

- Qian