Edited the repo's README

Converted README from Restructured Text to Markdown format
Edited entire README
Moved the ‘Git workflow’ content out into a wiki article and linked to it: https://github.com/GeoscienceAustralia/dea-notebooks/wiki/Git-workflow
Separated the ‘Introduction’ section into two sections: Browse the DEA Knowledge Hub and Use the DEA Sandbox
Added more links to the DEA Sandbox than on the current README
Fixed one broken link (tags index)
Commented out the tags content that is currently not relevant since the tags index isnt yet functional on the new Knowledge Hub.

I’ve spellchecked my work.

(Replace [ ] with [x] to check off)

[ ] Notebook created using the DEA-notebooks template
[ ] Remove any unused Python packages from Load packages
[ ] Remove any unused/empty code cells
[ ] Remove any guidance cells (e.g. General advice)
[ ] Ensure that all code cells follow the PEP8 standard for code. The jupyterlab_code_formatter tool can be used to format code cells to a consistent style: select each code cell, then click Edit and then one of the Apply X Formatter options (YAPF or Black are recommended).
[ ] Include relevant tags in the final notebook cell (refer to the DEA Tags Index, and re-use tags if possible)
[ ] Clear all outputs, run notebook from start to finish, and save the notebook in the state where all cells have been sequentially evaluated
[ ] Test notebook on both the NCI and DEA Sandbox (flag if not working as part of PR and ask for help to solve if needed)
[ ] If applicable, update the Notebook currently compatible with the NCI|DEA Sandbox environment only line below the notebook title to reflect the environments the notebook is compatible with
[ ] Check for any spelling mistakes using the DEA Sandbox's built-in spellchecker (double click on markdown cells then right-click on pink highlighted words). For example:

sandbox_spellchecker

GeoscienceAustralia / dea-notebooks