Add ODC Cheatsheet to intro docs

[robbibt]

Proposed changes

This is a simple PR to add the new Open Data Cube Cheatsheet reference doc to the ODC read-the-docs so that new users can more easily get started with datacube-core and odc.geo.

I've added a readme linking to the source doc if we need to make updates to the cheatsheet in the future.

The link to the cheatsheet (accessed by clicking on the thumbnail will not work in the preview docs, but should work once this PR is merged into develop.

• [ ] Closes #xxxx
• [ ] Tests added / passed
• [x] Fully documented, including docs/about/whats_new.rst for all changes

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📚 Documentation preview 📚: https://datacube-core--1543.org.readthedocs.build/en/1543/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0c5475e) 91.77% compared to head (de11d73) 91.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1543 +/- ## ======================================== Coverage 91.77% 91.77% ======================================== Files 132 132 Lines 14631 14631 ======================================== Hits 13428 13428 Misses 1203 1203 ```

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