Add pref guidance to desktop feature pref setting docs

[jaredlockhart]

Let's add the following guidance:

Guidance for using prefs in features:

1. Avoid using a pref in the first place and read directly from Nimbus API variables when possible. (as dwalker pointed out)
2. If you must use a pref, there should be no more than 1 writer per 1 pref (ie code, remote setting, UI in about:preferences, manual interaction in about:config). If a pref must be written to by multiple writers, consider using multiple prefs instead to maintain the 1 pref : 1 writer relationship.
3. Avoid guiding users to manually interact with and set prefs in about:config. This can weaken security practices by normalizing users to interact with complex 'under the hood' settings which can be taken advantage of by malicious actors. In addition, manually interacting with prefs that are managed by code can create confusing situations for users where their changes are overwritten or reverted.

to the top of the pref docs page:

https://experimenter.info/desktop-pref-experiments

[jaredlockhart]

Let's add 4. some guidance around how/when/where to use default or user branch.