Improve new user documentation experience

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (5069db7) 97.71% compared to head (60eaf2e) 73.49%.

:exclamation: Current head 60eaf2e differs from pull request most recent head d00a2c2. Consider uploading reports for the commit d00a2c2 to get more accurate results

Files	Patch %	Lines
hstrat/_auxiliary_lib/_cast_int_lossless.py	38.46%	8 Missing :warning:
..._phylogeny/_descend_template_phylogeny_alifestd.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #130 +/- ## =========================================== - Coverage 97.71% 73.49% -24.22% =========================================== Files 618 619 +1 Lines 8473 8489 +16 =========================================== - Hits 8279 6239 -2040 - Misses 194 2250 +2056 ``` | [Flag](https://app.codecov.io/gh/mmore500/hstrat/pull/130/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/mmore500/hstrat/pull/130/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno) | `73.49% <42.10%> (-24.22%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno#carryforward-flags-in-the-pull-request-comment) to find out more.

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[mmore500]

Excellent work here. Making a list of some remaining items. Depending on time, I might wrap them all up tomorrow morning if they're fast so we can put a bow on this.

A few notes,

• [x] one of the graphics is broken in the picking policy page (was it also already broken on README?)
• [x] we should take redundant content out of the readme and instead link to their new pages in docs
• [x] link quickstart from readme
• [x] check heading level of Using hstrat vs Quickstart --- only Quickstart should be showing in side panel
• [x] note hstrat version in quickstart

For myself,

• [x] line edit quickstart
• [x] revise Usage in README
• [x] add more direct discussion of why choosing retention policy is important to new doc page