Update docs for deprecated usage of Package::boot()

[tfrommen]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Update docs so they no longer include deprecated functionality.

What is the current behavior? (You can also link to an open issue here)

Currently, two pieces of documentation include the deprecated usage of Package::boot() with one or more module instances being passed to the method.

What is the new behavior (if this is a feature change)?

The documentation shows adding modules and booting the package in separate calls to addModule() and boot(), respectively.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

Other information:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8564405) 98.86% compared to head (f12590f) 98.86%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #39   +/-   ##
=========================================
  Coverage     98.86%   98.86%           
  Complexity      189      189           
=========================================
  Files             9        9           
  Lines           528      528           
=========================================
  Hits            522      522           
  Misses            6        6           

Flag	Coverage Δ
unittests	98.86% <ø> (ø)

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[Chrico]

LGTM. Thanks 💪🏻