Adjust wording in documentation to account for new projects

[imphil]

The documentation at https://www.lowrisc.org/docs/ is assuming that there is a single "lowRISC SoC". This is not the case any more with OpenTitan, which makes the documentation confusing to read.

We need to decide on

• How to name the "lowRISC SoC" project.

After these decisions, we need to

• Use that naming consistently across the web site.
• Rework existing documentation content to reflect that name/categorization.
• Think of a way to have documentation for our other projects next to it.