[docs] readthedocs builds are broken

[jameslamb]

Description

It seems the readthedocs have been broken for over a month (!).

With errors rendering the R docs.

✖ In topic 'lgb_predict_shared_params.Rd': Skipping; no name and/or title.
Error: 
! in callr subprocess.
Caused by error in `map2(.x, vec_index(.x), .f, ...)`:
! ℹ In index: 2.
Caused by error in `purrr::map()`:
ℹ In index: 5.
Caused by error:
! In '_pkgdown.yml', topic must be a known topic name or alias
✖ Not '`slice`'

(build link)

Reproducible example

Any recent build at

Environment info

N/A

Additional Comments

Based on the mention of slice, I suspect this might be related to #6293. But it must be one other thing, like a new version of {pkgdown} or something... because #6293 was merged February 3rd and the last successful docs build was March 19 (docs link). Suspect that #6346 also contributed to this.

[jameslamb]

I strongly suspect these are mainly the result of recent releases of {pkgdown}, which are now stricter.

In #6445, after fixing the error about slice(), I saw a new error:

Error: 
! in callr subprocess.
Caused by error in `check_missing_topics(rows, pkg)`:
! All topics must be included in reference index
✖ Missing topics: getLGBMThreads, lgb.configure_fast_predict, lgb.drop_serialized, lgb.make_serializable, lgb.restore_handle, lgb.slice.Dataset, print.lgb.Booster, setLGBMThreads, summary.lgb.Booster
ℹ Either add to _pkgdown.yml or use @keywords internal

After about 16 months without any releases, there were 2 new fairly-large ones pushed in the last month: https://github.com/r-lib/pkgdown/releases.