[docs] fix R documentation builds (fixes #3655)

[jameslamb]

This PR attempts to fix the broken R documentation (see #3655). Building the docs requires installing the R package. Now that #3405 has been merged, LightGBM's submodules have to be initialized for the package to be built successfully, and I think they were not on readthedocs builds.

latest build: https://readthedocs.org/projects/lightgbm/builds/12567523/

* checking for LF line-endings in source and make files and shell scripts
* checking for empty or unneeded directories
WARNING: directory ‘lightgbm/src/external_libs/fast_double_parser’ is empty
WARNING: directory ‘lightgbm/src/external_libs/fmt’ is empty

and then later

* installing to library ‘/home/docs/.conda/envs/r_env/lib/R/library’
* installing *source* package ‘lightgbm’ ...
** using staged installation
** libs
installing via 'install.libs.R' to /home/docs/.conda/envs/r_env/lib/R/library/00LOCK-lightgbm/00new/lightgbm
Building lib_lightgbm
In file included from /tmp/Rtmp4mfEea/R.INSTALL30a33e035de/lightgbm/src/include/LightGBM/config.h:16,
                 from /tmp/Rtmp4mfEea/R.INSTALL30a33e035de/lightgbm/src/include/LightGBM/boosting.h:8,
                 from /tmp/Rtmp4mfEea/R.INSTALL30a33e035de/lightgbm/src/src/boosting/boosting.cpp:5:
/tmp/Rtmp4mfEea/R.INSTALL30a33e035de/lightgbm/src/include/LightGBM/utils/common.h:35:10: fatal error: ../../../external_libs/fmt/include/fmt/format.h: No such file or directory
   35 | #include "../../../external_libs/fmt/include/fmt/format.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Notes for Reviewers

@StrikerRUS could you please enable readthedocs builds for this branch?

[StrikerRUS]

Thanks for the fix! I think it will be better to use config file for such things.
https://docs.readthedocs.io/en/stable/config-file/v2.html#submodules

Could you please try include the following lines

submodules:
  include: all
  recursive: true

into
https://github.com/microsoft/LightGBM/blob/master/.readthedocs.yml?

[StrikerRUS]

P.S. I've enabled RTD builds for the docs/jlamb branch and will keep it turned on as you'd asked.

[jameslamb]

sure, I can do that. And thanks for keeping the branch up!

[StrikerRUS]

And thanks for keeping the branch up!

This will auto-disable RTD builds! 🤣

[StrikerRUS]

Please use -1 for the consistency. It may help to simply a search query in the repo if we decide to change something in the future.

Shouldn't Rscript -e "pkgdown::build_site() cause builds to fail as well?

[jameslamb]

yeah you're right

[StrikerRUS]

You are right in the root cause of the failures! Builds are now passing:

[StrikerRUS]

Thanks for updating the config and forcing build to fail!

[github-actions]

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