Fix up weighted blending documentation

[brhooper]

Addresses https://github.com/metoppv/mo-blue-team/issues/497

Description
Moves document which describes the weighted blending method into /improver/doc/files/ and updates the link to that document within the weighted blending doc-string to refer to the new location.

Testing:

• Ran tests and they passed OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (27531d9) 98.37% compared to head (4b82dce) 98.38%.
Report is 5 commits behind head on master.

❗ Current head 4b82dce differs from pull request most recent head 218b2f7. Consider uploading reports for the commit 218b2f7 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1961   +/-   ##
=======================================
  Coverage   98.37%   98.38%           
=======================================
  Files         123      124    +1     
  Lines       11942    11976   +34     
=======================================
+ Hits        11748    11782   +34     
  Misses        194      194           

Files	Coverage Δ
improver/blending/weighted_blend.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bayliffe]

A request.

[bayliffe]

Suggested change

	/improver/doc/files/Combining_Probabilities.pdf
	:download:`Combining Probabilities by Caroline Jones, 2017 <../files/Combining_Probabilities.pdf>`

Could we make this downloadable in the external documentation with a link like this please?
The :download: bit makes readthedocs copy the file across to an accessible location. The path is relative to the source directory in the improver directory strucuture (improver/doc/source).

You cannot just commit this change as you need to delete the line above or it will have the title of the pdf twice.

You can see what that will look like here.

[brhooper]

Thanks for this - I've made the suggested change.