docs/dev/build_the_docs: use direct links to requirements.txt

The links previously used here were, in spite of having .txt at the end,
to respective GitHub HTML pages, which could come as a surprise to
someone who saved the links directly from the ReadTheDocs page and then
tried to feed the resulting files - with names still ending in .txt - to
pip. Using direct links both is less confusing and saves one click each,
regardless of whether one downloads the files or just passes links
directly to 'pip -r'.