content: source track draft: simplify and clarify level goals (#1097)

### Context

This was mostly ported from
[gdoc](https://docs.google.com/document/d/13Xt8mA_2b00McGX2vkyhu4GQdFAqtXPu7YXE8ZA6ISE/edit?resourcekey=0-EqfHF79tUWAKp4PzsE3z1A#bookmark=id.gg47kpxaq1to),
(requires
[slsa-discussion@googlegroups.com](mailto:slsa-discussion@googlegroups.com)
membership.)

The content is intentionally incomplete. 
The final draft document will need wholistic review before progressing
to the full proposal phase.

### Goals

The source track is about communicating trustworthy claims. 
These proposals for levels try to describe the absolute bare minimum
policies and controls required to make sense of the code in a repo.

This proposal moves most of the other "good idea" policies into a
different, non-leveled, section.
One of the goals of slsa is to help teams make improvements to their
process in a prioritized way.

Many of these good ideas should be called out and documented
_somewhere_, but they are not directly required for the repo to produce
trustworthy attestations, so we're proposing to document and discuss
them separately.

Update! As discussed [in
slack](https://openssf.slack.com/archives/C03NUSAPKC6/p1723156008871629?thread_ts=1723152271.940339&cid=C03NUSAPKC6),
products like the [ossf
scorecard](https://github.com/ossf/scorecard?tab=readme-ov-file#scorecard-checks)
might be better fits for describing policy details. The scorecard is
much more opinionated about things like branch protections already!

This pr addresses the topics raised in the following issues. 
We should re-valuate the status of these issues when this PR merges:
* #1076
* #1075 
* #1077
* #1095
* #1080

---------

Signed-off-by: Zachariah Cox <zachariahcox@github.com>
Signed-off-by: Tom Hennen <TomHennen@users.noreply.github.com>
Co-authored-by: Tom Hennen <TomHennen@users.noreply.github.com>
Co-authored-by: Aditya Sirish <8928778+adityasaky@users.noreply.github.com>