[image-processing-api] Improve documentation

[Salakar]

Documentation should be updated and improved;

• 'Text' operation has incorrect description and is missing the 'value' property; https://github.com/invertase/firebase-extensions/blob/main/extensions/image-processing-api/functions/src/operations/text.ts#L36
• Preview/example images not working (the deployed instance of the extension for demo purposes seemed to have been uninstalled.

[leeprobert]

I need more information about the text capabilities. In particular handling fonts. How do I get this? The documentation is non-existent at the moment.

[Salakar]

I need more information about the text capabilities. In particular handling fonts. How do I get this? The documentation is non-existent at the moment.

@leeprobert we do have docs; https://extensions.invertase.dev/image-processing-api/operations#text - however they seem to need a bunch of fixes and updates.

In terms of the 'text' operation, the code docs are up to date here: https://github.com/invertase/firebase-extensions/blob/main/extensions/image-processing-api/functions/src/operations/text.ts#L36 - you can specify a 'value' as well as the various styling properties, internally this will draw the text as an SVG and composite on top of your existing image