Added `tfPS()` function and some further touch-up

[LotharukpongJS]

• tfPS() function for quantile rank transformation of phylostratum values (as opposed to tf(), which transforms expression values). Since this was inspired by Lukas' notes, I added him as an author of the new function. The results should be equivalent to Julia function statsBase.quantilerank using method = :tied.
• reorganised the reference index in the website.
• updated the website bootstrap version to bootstrap v5 from v3 -- see discussion here.
• resolved some warnings when building the site using pkgdown::build_site(lazy = TRUE).

[HajkD]

Brilliant work as usual, @LotharukpongJS!

Thank you very much for adding this new function. This should be very useful for the evolutionary transcriptomics community.

Cheers, Hajk