add getTexture docs and examples for p5.strands

[nbogie]

Resolves #8438

Changes:

Add docs for getTexture in /src/strands/p5.strands.js including some examples. I tried to pick examples that don't need understanding of (m)any other shader functions.

Screenshots of the change:

Top section docs and first example:

Second example:

End section of docs (params, return value)

PR Checklist

• npm run lint passes - No, but my added code passes.
• [Inline reference] is included / updated

[nbogie]

I've shared this draft PR to get early feedback before converting the examples to the flat API

To-do:

• I think I need to re-write the examples using the "flat" API, following Experiment: flatter strands API #8314
• I need to fix the types.
• possibly, add one more example outside of a filter shader. This could follow later.
• add some description before each example (and possibly more comments)
• Find out if any interpolation is used when sampling from a lower-res texture, and if we have any control over that.

Questions:

• Should the examples use buildFilterShader() instead of modify()?
• How much should the function description clarify that it is for use in strands? (I can imagine this might be added as boilerplate for all functions in strands module)
• Is it necessary to bring my examples in line to be consistent with the others that pass an inline arrow function to modify() ? (I think that's significantly harder syntax for learners to read and debug, though more concise.)
• What should the param and return types be? I see no other functions referring to these vec2, vec4, sampler2D types.

current types

 * @param {typeof sampler2D} texture
 * @param {typeof vec2} coords
 * @returns {typeof vec4}

but these are all getting converted to any and void in type-gen.