[ci] [docs] docs builds broken: "In pkgdown/_pkgdown.yml, 1 topic missing from index: "lgb.interprete"

## Description readthedocs builds have been broken for a few days, with this error in the `{pkgdown}` site build: ```text Error: --   | ! in callr subprocess.   | Caused by error in `build_reference_index(pkg)`:   | ! In pkgdown/_pkgdown.yml, 1 topic missing from index: "lgb.interprete".   | ℹ Either add to the reference index, or use `@keywords internal` to drop from   | the index. ``` ([link to recent build](https://app.readthedocs.org/projects/lightgbm/builds/32629194/)) ## Reproducible example This is happening on readthedocs builds (https://app.readthedocs.org/projects/lightgbm/builds/32629194/) but I haven't yet observed it the job that renders the docs in regular PR CI. ## Notes Some things to look into: * new major version of `{roxygen2}` released about 10 days ago: https://cran.r-project.org/web/packages/roxygen2/) * `lgb.interprete()`-related changes about a month ago: #7148