Add section on add-on store to the Quick Start Guide in the User Guide

[Qchristensen]

Updated #16486 against md file rather than t2t file.

Link to issue number:

Closed #16399

Summary of the issue:

Request from users for more initial documentation on getting around the Add-on store.
This PR adds a section to the Quick Start Guide (in the user guide) on the add-on store.

Description of user facing changes

New sub-topic in the Quick Start Guide section of the User Guide "Add-on Store" after existing section 2.4 Preferences and before 2.5 Community.

Description of development approach

Testing strategy:

Known issues with pull request:

Code Review Checklist:

• Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• API is compatible with existing add-ons.
• Security precautions taken.

[Qchristensen]

Note line 287 about verifying add-ons prior to installation refers to the "Installing add-ons" section of the user guide and anticipates that either #16316 or #16510 will be implemented in the same release.

The base branch was changed.

[CyrilleB79]

One translator has noticed that in this PR, there are no blank line after the headings, on contrary to the rest of the User Guide. This does not seem to cause any change.

@Qchristensen or @seanbudd, could you confirm that the presence or not of blank lines after headings in .md files have no impact? Thanks.

[seanbudd]

as far as I am aware, these blank lines have no impact in markdown. this can be checked by ensuring the header and following paragraph text builds correctly to HTML