[blog] Add fixes and clean ups to the Blog page

[danilo-leal]

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

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

• 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]

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[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