Implement URL health check and update dead links

[Maspital]

Resolves #56

• Added a script which checks all kinds of links, you can run it via python tools/check_url_health.py from the repository root. Verifies the following:
  • internal relative links
  • internal absolute links
  • external links
  • markdown anchors
• Updated several dead links (where possible)
  • In the case of the three LANL datasets, their entire website seems to be down. I've left these up and will take another look in a couple weeks

[ru37z]

Reading the diff is really cumbersome due to the huge tables with changed whitespace. Is this really necessary or could you change those tables to just one delimiting space? The resulting web pages should be the same right? I know that the tables will look worse in the IDE then but it's well worth it IMHO.

[Maspital]

For me the diff looks like this, which I thought was pretty legible; does it look different for you?

Edit: Ah sorry, you meant the table in content.md.
Here, the large amount of changes stems from the fact that I switched to using VS Code for pretty much everything, which comes with the [GitHub Flavored Markdown] formatter. Here, tables are formatted as

| foo | bar |
| --- | --- |
| baz | bim |

Instead of

| foo | bar |
|-----|-----|
| baz | bim |

which caused a bunch of spaces to be added in a lot of places - this should be a one-time occurrence.
The only "significant" change is in line 37, where I corrected a filename typo.

In any case, if we want to change this, we should probably have a separate PR for this?

[Maspital]

By the way, all LANL links are working again, so good call on not changing that ^^