DOC-5601 Transform functions

[aimurphy]

Integrate transform functions content from streaming-leaning-docs into astra-streaming-docs:

• Add a transform functions reference page.
• Add transform function deployment info to the Deploy Functions in the Astra Portal section.
• Add subsection for sink deployment option.

[plpesvc-ds]

Build successful! ✅
Pull requests with matching branch name doc-5601 included in the build:

• DOC-5601 Transform functions #163 (this pull request)
• https://github.com/riptano/docs-common/pull/263
• DOC-5601 Remove functions topics streaming-learning-docs#71

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