Update prerequisites, update installation instructions, add Gitpod option

[rootwork]

This PR...

Changes / fixes

• Sets (in theme.toml) and documents a minimum Hugo version (0.91.0), because Hugo Clarity won't build prior to that version. (If we don't want to set a Hugo version that high, see the linked issue for what we would have to remove to make it possible for it to be lower.)
• Adds a Gitpod init file and suggests Gitpod as the first option for trying out the theme. I don't know how either of you feel about Gitpod; I realize there might be philosophical differences in using it. But I've seen other Hugo themes suggest this both for testing and contribution work, and I've used it myself in other open-source communities.
• Reorders the other installation options to put the Hugo modules version above the git submodules version. (Hu)go modules require an extra prerequisite but they're so much easier to maintain, I think that's what we should suggest first.
• Adds a piece in each option that removes the default config.toml that gets created in the root directory when you run hugo new site. Given that Hugo Clarity provides a config directory if you copy everything over from exampleSite, I think it might be confusing to people to have both.
• Makes it clear why exampleSite is copied and what it provides, and suggests an alternative if adding this theme to an existing site.
• Along the way, corrects a few mistakes. I tried each of the installation options and there were a few typos in what was there previously.
• I changed the link that pointed to the "Hugo modules for dummies" article to my example Hugo modules site. I hope that's not impetuous; I'm genuinely trying to be helpful with that example, and I include the "dummies" article in the list of resources at the bottom.
• Notes at the top of the configuration section that Hugo Clarity uses a config directory rather than a single config file.

Question

If you agree that Hugo modules should be suggested first over git submodules, should we also change the theme value in exampleSite/config/_default/config.toml to the Hugo module version? That would make that option even easier, and would put the burden of opening the file and changing that value on people who want to use git submodules instead.

Screenshots (if applicable)

Just read from https://github.com/chipzoller/hugo-clarity/tree/prereq-install-updates#prerequisites down to the top of the configuration section.

Checklist

Ensure you have checked off the following before submitting your PR.

• [ ] tested locally with the latest release of Hugo. This requirement is a standard
• [ ] added new dependencies
• [x] updated the [docs]() ⚠️

[rootwork]

Oh, if you want to try the Gitpod integration, here's the temporary URL:

https://gitpod.io/#https://github.com/chipzoller/hugo-clarity/tree/prereq-install-updates

If this PR gets merged, than the URL would just be https://gitpod.io/#https://github.com/chipzoller/hugo-clarity which is what the proposed documentation links to.