Set R >=4.6.0's help.htmltocoption to FALSE

[DavisVaughan]

Addresses posit-dev/positron#12840

Relevant R commits:

• wch/r-source@2022aa9
• wch/r-source@3484008

R 4.6.0 added a new help.htmltoc global option. The point of this was to help add a TOC on the left hand side of CRAN's new HTML page for R package docs: https://cran.r-project.org/web/packages/vctrs/refman/vctrs.html

It's on for "dynamic help", i.e. when you go through httpd() which calls Rd2HTML(dynamic = TRUE, toc = getOption("help.htmltoc", TRUE)), and we of course use httpd() for installed package help.

(Notably its not broken for "preview" R package help, i.e. for development version docs, since we control the Rd2HTML() call there and don't supply toc, see .ps.Rd2HTML()).

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We have two options:

• Turn it off using the only hook we have, the help.htmltoc global option
• Try to use it, by adapting to the new R-nav.css stylesheet, which weirdly feels like R.css with some extra additions? https://github.com/wch/r-source/blob/d5e28ead42751bc5a46bef44b3f351642c4d60d3/doc/html/R-nav.css#L2-L6

I think trying to use R-nav.css is probably a mistake, the TOC won't get us much, and is supposed to be hidden on narrow screens anyways. Since the Help pane is usually narrow, it would pretty much always be hidden.

So instead the best option is to just turn it off on load.

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I have confirmed locally that this fixes the mangled help issue.