Add relevant files and folders for the dldoc branch

[Anirban166]

dldoc can be run on this branch, with similar results as observed in Anirban166/GSoC21-Tests#9 (comment)

Steps to reproduce the changes included here:

• Download the svn repo from the scm page of directlabels (https://r-forge.r-project.org/scm/?group_id=481) using:

svn checkout svn://svn.r-forge.r-project.org/svnroot/directlabels/

• Apart from the doc folder within pkg/directlabels/tests/, Delete everything inside pkg/directlabels/ and emplace the contents of the current directlabels master branch within the same.
• Add prostate.Rdata within data/ and change instances of data(prostate,package="ElemStatLearn") to data(prostate) to avoid loading from the currently unavailable "ElemStatLearn" package (throws error, as seen here), and instead directly load from the Rdata file. I made this change for the concerned files (tests/doc/lineplot/lars and tests/doc/lineplot/ridge) within the gh-pages branch too.