Issue 1955 and 2028

[anisa-hawes]

I am updating en/lessons/building-static-sites-with-jekyll-github-pages.md and es/lecciones/sitios-estaticos-con-jekyll-y-github-pages.md to improve the advice for users who encounter a permissions error after they attempt gem install rubygems-update. I am also adding an alert warning to indicate that users of macOS Catalina and Big Sur may encounter difficulties in this lesson.

Closes #1955 and Closes #2028

Checklist

• Assign yourself in the "Assignees" menu
• Assign at least one individual or team to "Reviewers"
  • if the text needs to be translated, assign the relevant language team(s) as "Reviewers" and tag both the team as well as the managing edtor in your PR. Please follow the translation request guidelines when writing your PR description
• Add the appropriate "Label"
• Ensure the status checks pass
• Check the live preview of your PR on Netlify
• If this PR closes an open issue, add the phrase Closes #ISSUENUMBER to the description above

If you are having difficulty fixing build errors, first consult https://github.com/programminghistorian/jekyll/wiki/Making-Technical-Contributions carefully, especially "Common Build Errors". Then contact the technical team if you need further help.

[anisa-hawes]

Thank you for your help today @rivaquiroga! ✨