Fix ReadtheDocs PR build failure

[WeinaJi]

Context

After enabling the PR build for ReadtheDocs, we got doc build failure such as:

WARNING: Summarised items should not include the current module. Replace 'neurodamus.Neurodamus' with 'Neurodamus'. [autosummary.import_cycle]
WARNING: Summarised items should not include the current module. Replace 'neurodamus.Node' with 'Node'. [autosummary.import_cycle]
WARNING: Summarised items should not include the current module. Replace 'neurodamus.cell_distributor' with 'cell_distributor'. [autosummary.import_cycle]
...

This PR fixes the relevant doc files according to those warnings.

Testing

ReadtheDocs PR build

Review

• [x] PR description is complete
• [ ] Coding style (imports, function length, New functions, classes or files) are good
• [ ] Unit/Scientific test added
• [x] Updated Readme, in-code, developer documentation

[bbpbuildbot]

Logfiles from GitLab pipeline #235513 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: docs
• :white_check_mark: test-unit
• :white_check_mark: test-integration
• :white_check_mark: test-integration-e2e
• :white_check_mark: test-scientific
• :white_check_mark: test-scientific-ngv
• :white_check_mark: set_alt_branches
• :white_check_mark: build-package
• :gear: publish-package
• :gear: publish-docs