Tools article should give instructions for how to include custom tools

[gadenbuie]

https://posit-dev.github.io/querychat/r/articles/tools.html

There's also a brief mention of providing a direct ellmer client for "advanced use cases (e.g. custom parameters, tools, etc.)" in https://posit-dev.github.io/querychat/r/articles/models.html#specify-a-model