Fix rendering of docs

[JCGoran]

This should take care of the rendering of the docs, which was caused by a combination of two things:

1. the (now deleted) line was overriding /index.html file, and was redirecting it to /html/index.html instead
2. where does the /html/index.html file come from? I believe the answer is this line here: https://github.com/BlueBrain/nmodl/blob/3754a71af34180633bb7261fd52648f277fd93f7/.github/workflows/nmodl-doc.yml#L82

At some point I guess the docs were (and still are) under /html/[file] instead of just /[file].

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.54%. Comparing base (3754a71) to head (7e9b3f0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1302 +/- ## ======================================= Coverage 85.54% 85.54% ======================================= Files 178 178 Lines 13415 13415 ======================================= Hits 11476 11476 Misses 1939 1939 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #215374 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: spack_setup
• :white_check_mark: build:intel
• :white_check_mark: build:nvhpc
• :white_check_mark: test:intel
• :white_check_mark: test:nvhpc