Restore building of documentation downloads

Although documentation resumed being built in 634151a, and the RTD
theme and API Reference section were restored in 64ad585 (#1843),
documentation for download did not resume being built, with 3.1.37
(not 3.1.42) being the latest version listed here:

    https://readthedocs.org/projects/gitpython/downloads/

Three kinds of downloadable documentation are supported -- PDF,
ePub, and HTML -- and all three were previously being built but
all have stopped.

This attempts to resume building all three, using `all` as the
value of the `formats` key in .readthedocs.yml.

A string value of `all` currently should have the same effect as a
sequence value of the strings `htmlzip`, `pdf`, and `epub`. (In the
future, `all` may build more formats.) See:

- https://docs.readthedocs.io/en/stable/downloadable-documentation.html
- https://docs.readthedocs.io/en/stable/config-file/v2.html#formats

.readthedocs.yaml

@@ -23,9 +23,7 @@ sphinx:
   fail_on_warning: true
 
 # Optionally build your docs in additional formats such as PDF and ePub.
-# formats:
-# - pdf
-# - epub
+formats: all
 
 # Optional but recommended, declare the Python requirements required
 # to build your documentation.