Fix bug when building docs locally

[dachengx]

Before you submit this PR: make sure to put all XENONnT specific information in a wiki-note as the repo is publicly accessible

What does the code in this PR do / what does it improve?

Before this PR, when running

make html

in folder fuse/docs, error shows:

Exception occurred:
  File "/opt/miniconda3/envs/XENONnT_2024.03.1/lib/python3.9/shutil.py", line 722, in rmtree
    orig_st = os.lstat(path)
FileNotFoundError: [Errno 2] No such file or directory: '/home/xudc/fuse/docs/source/graphs'

this is because the "graphs" folder is not defined correctly in shutil.rmtree

Can you briefly describe how it works?

Define a unified path of graphs.

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• [ ] Update the docstring(s)
• [ ] Bump plugin version(s)
• [ ] Update the documentation
• [ ] Tests to check the (new) code is working as desired.
• [ ] Does it solve one of the GitHub open issues?

[coveralls]

Pull Request Test Coverage Report for Build 9088401751

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.506%

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Totals
Change from base Build 8936992735:	0.0%
Covered Lines:	2108
Relevant Lines:	2948

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💛 - Coveralls