New content for mmrm pages

[yonicd]

Hi @statasaurus @DrLynTaylor this PR is for the content re the mmrm.

Let me know if any formatting or content changes should be made

[yonicd]

Not sure why, but {knitr} is getting 404 when trying to install from CRAN, which is causing build to fail.

[DrLynTaylor]

Not sure why, but {knitr} is getting 404 when trying to install from CRAN, which is causing build to fail.

@yonicd Thank you so much for your quick work. I think this is the same issue Chi & I got when trying to render. It could be something to do with the renv file. @statasaurus when you are back from the conferences this week, perhaps you, me, @andreaczhang and @yonicd could meet up, check the build & content and then we can get this MMRM work included in the repo.

[statasaurus]

Cause there were a few other pull requests with renv issues so I just rolled them together into a pull request. But, the update is now on the website. Thank you so much for this contribution!!

[yonicd]

@statasaurus it looks like the content that was here didnt make it to the site (specifically R/mmrm.R)

[statasaurus]

let me add that in thank you for raising this

[statasaurus]

Sorry for this taking so long, I have now added the file