Check for broken links

@Fienne found a broken link in the user guide. That reminded me of two tools that can be used for finding issues like this. The second can also be used to prevent, by hooking it up in our CI system.

1. W3 checklink: https://validator.w3.org/checklink

It checks anchors, internal, and external links. You need to provide the parameters that can be saved in a cookie locally. It takes a few seconds/minutes, but gives you a pretty detailed report to go through and fix any broken links.

2. Sphinx linkcheck

Not sure if it can be configured to inspect external links. It checks anchors and internal links. Comes with Sphinx. It is a builder like the `html` builder. Here's an example run, grep'ing for the token "broken":

```bash
$ sphinx-build -b linkcheck src _build/ | grep broken
(introduction/prerequisites: line 58) broken https://github.com/common-workflow-language/cwltool#install - Anchor 'install' not found
(topics/specifying-software-requirements: line 25) broken https://github.com/common-workflow-language/cwltool#leveraging-softwarerequirements-beta - Anchor 'leveraging-softwarerequirements-beta' not found
(topics/file-formats: line 45) broken https://github.com/common-workflow-language/user_guide/raw/main/_includes/cwl/file-formats/file-formats.bam - 404 Client Error: Not Found for url: https://github.com/common-workflow-language/user_guide/raw/main/_includes/cwl/file-formats/file-formats.bam
(introduction/prerequisites: line 173) broken https://github.com/rabix/benten#install-vs-code-extension - Anchor 'install-vs-code-extension' not found
(topics/custom-types: line 24) broken https://raw.githubusercontent.com/common-workflow-language/user_guide/main/_includes/cwl/custom-types/rich_sparse_otu_table.biom - 404 Client Error: Not Found for url: https://raw.githubusercontent.com/common-workflow-language/user_guide/main/_includes/cwl/custom-types/rich_sparse_otu_table.biom
(introduction/prerequisites: line 176) broken https://www.commonwl.org/#Editors_and_viewers - Anchor 'Editors_and_viewers' not found
```

I did not check if the links are indeed broken. But we used it in the Cylc project (I believe they still use it, and for external links? → https://github.com/cylc/cylc-doc/blob/5a5aaa111c6756fdf7c4ab9b2f8ec6cabd37ae6e/src/conf.py#L136-L140).

@swzCuroverse maybe a good one for Outreachy? A little harder to work on this as it requires some researching into how Sphinx works (although it should be nerdily fun! :nerd_face: ).

-Bruno