[docs] switch vignettes from 'rmarkdown' to 'markdown'

[jameslamb]

Description

This project has one small vignette that doesn't rely on any special features of R markdown.

{uptasticsearch} should switch its rendering engine from {rmarkdown} to {markdown}, to reduce the number of dependencies that need to be installed to build the docs.

Benefits of this work

• reduces the maintenance effort required to keep the package on CRAN
  • e.g., reduces the risk of being affected by stuff like with_mock() is going away #223
• reduces the burden that {uptasticsearch} places on other projects and on CRAN
  • by removing {uptasticsearch} from more projects' reverse dependency lists

Acceptance criteria

• {uptasticsearch}'s docs can be built without {rmarkdown}

Approach

See microsoft/LightGBM#6258

Notes

N/A