docs: Add info about Pygments installation in installing.md
docs: Add info about Pygments installation in installing.md

Additional information about the Pygments dependency in the "installing from source" chapter.
mdhender/hugo,gohugoio/hugo,fedelibre/hugo,mdhender/hugo,mitchchn/hugo,mitchchn/hugo,anthonyfok/hugo,digitalcraftsman/hugo,mdhender/hugo,fedelibre/hugo,anthonyfok/hugo,fedelibre/hugo,mdhender/hugo,anthonyfok/hugo,digitalcraftsman/hugo,gohugoio/hugo,gohugoio/hugo,fedelibre/hugo