Add xcube tutorial

[konstntokas]

I added the xcube.ipynb to the tutorials which shows an example of using xcube to lazily load Sentinel-2 data from SentinelHub and calculate the spectral indices on the fly using spyndex.

There are few things to add:

• add colab batch
• add 'level 9 xcube' image

@sonicious Can you help me with these two remaining ToDos? Thanks

[Sonicious]

The colab is actually a link. If you check the raw file of the planetary computing, you see it properly:

{
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "[![colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/davemlz/spyndex/blob/main/docs/tutorials/pc.ipynb)\n",
    "![level7](https://raw.githubusercontent.com/davemlz/spyndex/main/docs/_static/level8.png)"
   ]
  }

The graphics were made by @davemlz I guess he has the prototypes to create a new one for you.

[konstntokas]

The colab is actually a link. If you check the raw file of the planetary computing, you see it properly:

{
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "[![colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/davemlz/spyndex/blob/main/docs/tutorials/pc.ipynb)\n",
    "![level7](https://raw.githubusercontent.com/davemlz/spyndex/main/docs/_static/level8.png)"
   ]
  }

The graphics were made by @davemlz I guess he has the prototypes to create a new one for you.

The I guess @davemlz also needs to add the xcube.ipynb to https://colab.research.google.com/github/davemlz/spyndex/blob/main/docs/tutorials, right?

[davemlz]

Earth Engine test failed. But that's ok. It's because of the auth token. I have it in main here. I'm merging ;)