title: Installing package fails when building vignettes ((..)/doc/index.html is missing)
body: <p>I'm creating an R package with a vignette. However, when I try to install the package by using</p>
  
  <pre><code> devtools::install(build_vignettes = TRUE)
  </code></pre>
  
  <p>an error occurs since it cannot find the file index.html in the doc folder. I thought this file would be automatically created, but apperently something goes wrong here. The error occuring is the following:</p>
  
  <pre><code>"C:/PROGRA~1/R/R-34~1.3/bin/x64/R" --no-site-file --no-environ --no-save --no-restore  \
  --quiet CMD INSTALL "C:/Users/lnab/AppData/Local/Temp/Rtmpi0XLJ4/mecor_0.1.0.tar.gz"  \
  --library="\\vf-d2-home/d2home$/lnab/MyDocs/R/win-library/3.4" --install-tests 
  
  * installing *source* package 'mecor' ...
  ** R
  ** data
  *** moving datasets to lazyload DB
  ** inst
  ** preparing package for lazy loading
  ** help
  *** installing help indices
  converting help for package 'mecor'
  finding HTML links ... done
  mecor-package                           html  
  uaetrial                                html  
  uaetrial_cal                            html  
  ** building package indices
  ** installing vignettes
  Warning in file(con, "w") :
  cannot open file '\\vf-d2-home/d2home$/lnab/MyDocs/R/win-library/3.4/mecor/doc/index.html': No such file or directory
  Error in file(con, "w") : cannot open the connection
  ERROR: installing vignettes failed
  * removing '\\vf-d2-home/d2home$/lnab/MyDocs/R/win-library/3.4/mecor'
  * restoring previous '\\vf-d2-home/d2home$/lnab/MyDocs/R/win-library/3.4/mecor'
  In R CMD INSTALL
  </code></pre>
  
  <p>My vignette .Rmd looks like this:</p>
  
  <pre><code>---
  title: "Introduction to mecor"
  author: "Linda Nab"
  date: "`r Sys.Date()`"
  output: rmarkdown::html_vignette
  vignette: &gt;
    %\VignetteIndexEntry{Introduction to mecor}
    %\VignetteEngine{knitr::rmarkdown}
    %\VignetteEncoding{UTF-8}
  ---  
  </code></pre>
  
  <p>I never build an R package before but search the internet for a solution for this problem for days but cannot find any. <a href="https://github.com/hadley/devtools/issues/705" rel="noreferrer">This</a> seems to be the same problem but I cannot find the solution for the problem there. You can find my package <a href="https://github.com/LindaNab/mecor" rel="noreferrer">here</a>.</p>
  
  <p>Really hope someone could help me! </p>