Add a walkdocs function to walk over all HTML files of a docs directory

[mortenpi]

It's a pretty common operation to want to do some operation on every HTML file of a Documenter build. E.g. to figure out the sizes of all HTML files:

filesizes = DocumenterTools.walkdocs("docs/build"; collect=true) do fileinfo
    fileinfo.relpath => filesize(fileinfo.fullpath)
end