Add initial Literate documentation

[JeffFessler]

I am looking for an incomplete Cholesky decomposition precoditioner and I found this package. To help others (and myself) figure out how to use this code some docs would be helpful. Here is a PR with at least some basics.

[mohdibntarek]

I don't know Literate enough to review this. But it seems CI isn't giving a preview of the docs.

[JeffFessler]

But it seems CI isn't giving a preview of the docs.

I forgot to include the CI for the docs. It is added now. There are settings to force the CI to make a preview of the docs but I don't usually use that because it seems to make the repo get larger. I check the preview locally by running julia --project=. make.jl in the docs/ directory. But to make it easier for you, for now I changed the one line in make.jl to be push_preview = true so now it should build a doc preview.

The docs are not too exciting now because this package has so few docstrings...

[JeffFessler]

BTW, the docs will deploy only if someone who is a maintainer enables github pages with appropriate ssh deploy keys under Settings tab. See https://documenter.juliadocs.org/stable/man/hosting/#GitHub-Actions

[JeffFessler]

I tried to manually create a gh-pages branch at the command line and push that. Then I reran the CI. The CI ran, but did not create any preview in the gh-pages branch. I think it can work only if the maintainers set that up under Settings, and then rerun the workflow.

[mohdibntarek]

I setup the keys. Let's see.

[mohdibntarek]

You can now preview the docs at https://julialinearalgebra.github.io/Preconditioners.jl/previews/PR31

[mohdibntarek]

Now it also shows up in the checks under documenter/deploy which you can click to preview the docs.

[JeffFessler]

I see that it is running all the CI tests, even when only docs are changed. Eventually the CI file should be updated to prevent that...

[JeffFessler]

I forgot to update the README with links to the docs. It will be a while before I have time to do that though.