Clarify which fns depend on external tools (ollama)

[mpadge]

From @Selbosh's review:

In the long run, I'd suggest clearly marking out which functionality is dependent on which external tools, as then a new user might be able to explore some features even if they have issues with others or do not want to set them up yet.

[mpadge]

Plan is now to enable most functionality to work without ollama, through reverting to BM25 matches only, and issue an accompanying warning.

[mpadge]

Everything now works without ollama, and just issues warnings like this:

library (pkgmatch)
#> ollama is installed but not running. Please run `ollama serve` from a separate console (not from within R).
input <- "Package that works with dates and times in tidy format"
pkgmatch_similar_pkgs (input, corpus = "cran")
#> Warning: ollama is not available. Matches will be based on word frequencies only, and
#> will generally be inferior to results enhanced with ollama. Run
#> 'ollama_check()' for diagnostic help, or see vignette on setting up ollama.
#> [1] "sweep"    "cleaner"  "timeDF"   "autoTS"   "timeplyr"

^{Created on 2025-07-08 with reprex v2.1.1}