docs: add link to sprig website in the template docs site

[KyriosGN0]

added documation that the Spring function are also availabe in the templates section of the notification docs

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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[nitishfy]

Minor nit added

[KyriosGN0]

@nitishfy thx for the review, i committed your suggestion