feat: generate sitemap.xml (resolves #243)

[jobara]

• This pull request has been linted by running npm run lint without errors
• This pull request has been tested by running npm run start and reviewing affected routes
• This pull request has been built by running npm run build without errors
• This isn't a duplicate of an existing pull request

Description

Steps to test

1. Perform a build: npm start or npm run build

Expected behavior: The sitemap.xml and robots.txt files should be generated at the root of the site.

Additional information

This PR also removes the /admin/ and 404.html pages from indexing.

Related issues

Resolves #243

[greatislander]

Looks great!