make default timezone configurable and doc

[pindge]

closes #381

• Due to loading order, flask application is loaded after the summarise code, the timezone configuration needs to be from os level.

Scope

• add doc
• make _summarise timezone configurable

---

:books: Documentation preview :books:: https://datacube-explorer--529.org.readthedocs.build/en/529/

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (86509bc) 86.51% compared to head (7a3d96c) 86.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #529 +/- ## ======================================== Coverage 86.51% 86.51% ======================================== Files 26 26 Lines 3440 3441 +1 ======================================== + Hits 2976 2977 +1 Misses 464 464 ``` | [Impacted Files](https://codecov.io/gh/opendatacube/datacube-explorer/pull/529?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube) | Coverage Δ | | |---|---|---| | [cubedash/summary/\_summarise.py](https://codecov.io/gh/opendatacube/datacube-explorer/pull/529?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube#diff-Y3ViZWRhc2gvc3VtbWFyeS9fc3VtbWFyaXNlLnB5) | `96.15% <100.00%> (+0.04%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube)

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