fix: repetitive YouTube embeds and utterances sections in each page

[digital-carver]

A lot of the pages have repeated embeds of the same YouTube videos - for eg., the INTRO page https://sylvaticus.github.io/SPMLJ/stable/00_-_INTRO_-_Introduction_julia_ml/0003_-_Introduction_to_Julia.html has six copies of the same div containing the same "Julia overview" video embed.

Similarly, the utterances comment section is also shown multiple times in each page because it's div also has redundant copies in the HTML.

This PR removes the repeated copies, leaving one youtube embed div and one utterances section in each case as it should be.

[digital-carver]

I see now that these sections are generated in code in https://github.com/sylvaticus/SPMLJ/blob/main/makedoc.jl . So fixing this in the markdown is not the solution I guess?

I've not looked at the code too deeply, but maybe the problem is when doing outContent *= origContent at

SPMLJ/makedoc.jl

Line 196 in 8bb0257

outContent *= origContent

, if the origContent already has a YouTube embed, this step adds another redundant one? So every time the docs are built, another copy of the YouTube embeds div and utterances div are added in? That seems a likely explanation for why these pages end up this way.