[blog] Add the "Product" tag to the Pigment post + other clean ups

[danilo-leal]

• [x] I have followed (at least) the PR section of the contributing guide.

A quick run-through of the changes in this PR:

• Add the "Product" tag to the latest Pigment CSS post
• Remove the "MUI System" tag (not used)
• Fix the color of the "+2" (i.e., additional authors) number in the Avatar Group component
• Fix the HTML element on the complementary info box on the right-hand side column
• Clean up small bits of the code

[mui-bot]

Netlify deploy preview

https://deploy-preview-42311--material-ui.netlify.app/

Bundle size report

No bundle size changes (Toolpad) No bundle size changes

Generated by :no_entry_sign: dangerJS against 4a1839f6f4e626f402e78ea40607ec32fe0e0081

[DiegoAndai]

Hey @danilo-leal! It would be better to split this PR into two:

This can be cherry-picked (blog content specific):

• Add the "Product" tag to the latest Pigment CSS post

This should stay on next until it becomes master (blog infra):

• Remove the "MUI System" tag (not used)
• Fix the color of the "+2" (i.e., additional authors) number in the Avatar Group component
• Fix the HTML element on the complementary info box on the right-hand side column
• Clean up small bits of the code

This will help minimize merge conflicts and overhead tracking next <> master diffs