Make sure all v6 features are documented

[tabacitu]

To document:

• installation process now asks the developer which theme they want to install;
• installation process currently favors installing CoreUIv2, so we can't say Tabler is the default just yet;
• UPGRADE GUIDE - note all version of our packages that support Backpack v6, so it's easy for people to bump the version;
• UPGRADE GUIDE - shouldn't we tell people to change all their custom pages to extend backpack_view('blank') because that's provided by all themes?
• UPGRADE GUIDE: you should run php artisan backpack:install to properly install Basset and the theme;
• UPGRADE GUIDE - shouldn't we tell people to delete their errors folder? if they haven't customized it with their own error views of course;
• UPGRADE GUIDE - in v6 we define the uploaders in the crud.php config file; do we instruct people to add these in the upgrade guide? what happens if they don't do it? nothing; they will add them when they want to create custom uploaders; which is never;
• UPGRADE GUIDE - in v6 all operations content class defaults to being wide (col-md-12); should we instruct people to do that in their upgrade guide? nah... let's let them keep their old width;
• CrudButton now has metas and you can send variables to your buttons; is this documented anywhere?
• CrudColumn is Macroable now; is that documented anywhere? should we?
• CrudField is Macroable now; is that documented anywhere? should we?
• are all Uploaders documented?
• are all Uploaders referenced where they're most likely to be needed?
• do our general-purpose docs and examples show Uploaders instead of mutators or model events?
• we now have a few custom validation rules; are they well documented?
• in v6 we add a way to quickly add a button - is that properly documented?
• i'm pretty sure the bp-section classes we've added aren't documented at all; see this as an example;
• we should document how someone uninstalls a Backpack theme; for ex in the upgrade process we instruct them to use CoreUIv2, and only after everything is workding... install Tabler and uninstall CoreUIv2; but we don't explain how to do that (uninstall);
• currently our docs don't do a good job at highlighting how much of a team effort Backpack v6 was; we MUST give credit where credit is due; we must mention our team members by name; and thank them for their efforts;
• we should mention how much work has gone into v6, with numbers:
  • CRUD alone has had more than 614 commits;
  • PRO has had more than 221 commits;
• our drip emails for "getting started" have not been updated for v6; at all!
• spatie media library support is nowhere in the docs, only in the release notes;
• The "How To" Page got a lot of old info which is not usable in v6 #488
• CSS Hook section is missing in docs. #484

[tabacitu]

• polish all pages that were noted here as not final:

[tabacitu]

[pxpm]

I stared at this for a moment thinking you were reporting a bug because the file seems to "disapear" 🤷 funny!