Converting a whole website may be a lot of work.
Use a copy of the mirror.
Page by page, you can select all, then copy and paste into
an editor and save as text (only text will be kept).
You can also use Arachnophilia for example (Windows version
4 for example) then open your files, Edit/Select all, then
Selection/Strip all HTML tags and save as a text file (but
javascript and css will not be removed),
then use Arachnophilia search and replace in all open docs
facility to remove js and css.
You can also find a few freeware doing the same, but file by
file and leaving javascript and css in the processed file.

If you master regular expressions, many search and replace
utilities (such as Handy Find and Replace) will do the job
in the whole website quite rapidly.