[website] Write new Installation doc

[jackkoenig]

There's a lot more I'd like to do, but I also want to avoid letting the perfect be the enemy of the good / scope creep and this vastly improves the getting started experience for the website.

This adds a new Getting Started section with an Installation page with a nice Getting Started section using Scala CLI

As a teaser for how I want to make it even easier to get started, I have tweaked the chisel-template in a way that makes it super easy to start a new project, see https://github.com/jackkoenig/chisel-template.g8 which I intend to write a docs page for to put under Installation, something like Starting a New Project.

Screenshots showing off how things look, note new Getting Started link front and center on homepage:

Clicking that link takes you to the new Installation page with it's Quickstart:

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Documentation or website-related

Desired Merge Strategy

• Squash

Release Notes

Revamps the website's instructions for "Getting Started".

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.5.x, 3.6.x, or 5.x depending on impact, API modification or big change: 6.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[seldridge]

Looks great! This was missing for a long time. Great initiative, Jack!

[seldridge]

Advising uses to install something by moving things into a root-owned location is questionable. However, this is also what scala-cli suggests.

[jackkoenig]

AFAICT it's pretty universal that Linux-y things suggest you install things in /usr/local/bin and if there's a reason you shouldn't it seems that the reader is expected to know this and adjust accordingly. I don't have a strong opinion, I'm just following typical style.