### Description
To provide clear, accessible documentation for each of these pre-built
tools, add a new heading for `Pre-built Tools` under the "Available
Tools" section on each Source page.
### Related Issues
Address and resolves#960
---------
Signed-off-by: Anushka Saxena <anushkasaxenaa@google.com>
Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>
Environment variable replacement is needed so that users don't have to
hardcode their secrets in configuring `tools.yaml`.
Both formats `$ENV_NAME` and `${ENV_NAME}` are standard ways to declare
an environment variable.
However, some database statement placeholders that are already using the
`$ENV_NAME` format.
Therefore, we only support env var declaration using `${ENV_NAME}` to
disambiguate it from other usages.
Fixes issue: https://github.com/googleapis/genai-toolbox/issues/431
