Correct links to Eijkhout textbooks

[VictorEijkhout]

The pages.tacc links in the "advaced" section have gone poof.

Please list as a general link: https://theartofhpc.com/ and separate links for

• Intro to HPC https://theartofhpc.com/istc.html

• Parallel programming https://theartofhpc.com/pcse.html

• HPC carpentry https://theartofhpc.com/carpentry.html

[georgiastuart]

In addition:

• [ ] add link check step to build process.

[georgiastuart]

Links fixed in a3f38a1. Leaving open for the link checker.