This extends the page grouping in Hugo with a template function that allows for ad-hoc grouping.
A made-up example:
```
{{ $cool := where .Site.RegularPages "Params.cool" true | group "cool" }}
{{ $blue := where .Site.RegularPages "Params.blue" true | group "blue" }}
{{ $paginator := .Paginate (slice $cool $blue) }}
```
Closes#4865
This also adjusts the pagination logic to allow for these new collections.
Note that we will follow up with a template function named `group` that will be the end user API. The `.Group` method on `Page` should be considered as internal.
Updates #4865
Fix godoc comments and the following:
resource/image.go:33:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:35:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:41:2: a blank import should be only in a main or test package, or have a comment justifying it
resource/image.go:538:10: if block ends with a return statement, so drop this else and outdent its block
resource/resource.go:54:7: exported const DefaultResourceType should have comment or be unexported
resource/resource.go:233:1: receiver name r1 should be consistent with previous receiver name r for Resources
resource/resource.go:254:1: receiver name r1 should be consistent with previous receiver name r for Resources
related/inverted_index.go:76:1: exported method Config.Add should have comment or be unexported
related/inverted_index.go:255:1: exported method IndexConfig.ToKeywords should have comment or be unexported
helpers/general.go:448:1: comment on exported function DiffStrings should be of the form "DiffStrings ..."
helpers/hugo.go:42:6: exported type HugoVersionString should have comment or be unexported
helpers/hugo.go:48:1: exported method HugoVersion.Version should have comment or be unexported
helpers/hugo.go:56:1: comment on exported method HugoVersionString.Compare should be of the form "Compare ..."
helpers/hugo.go:62:1: comment on exported method HugoVersionString.Eq should be of the form "Eq ..."
helpers/path.go:548:1: comment on exported function OpenFilesForWriting should be of the form "OpenFilesForWriting ..."
helpers/processing_stats.go:24:6: exported type ProcessingStats should have comment or be unexported
helpers/processing_stats.go:55:1: exported function NewProcessingStats should have comment or be unexported
helpers/processing_stats.go:59:1: exported method ProcessingStats.Incr should have comment or be unexported
helpers/processing_stats.go:63:1: exported method ProcessingStats.Add should have comment or be unexported
helpers/processing_stats.go:67:1: exported method ProcessingStats.Table should have comment or be unexported
helpers/processing_stats.go:83:1: exported function ProcessingStatsTable should have comment or be unexported
deps/deps.go:79:6: exported type Listeners should have comment or be unexported
deps/deps.go:86:1: exported method Listeners.Add should have comment or be unexported
deps/deps.go:92:1: exported method Listeners.Notify should have comment or be unexported
config/configProvider.go:54:9: if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary)
config/privacy/privacyConfig.go:99:1: exported function DecodeConfig should have comment or be unexported
config/services/servicesConfig.go:64:1: exported function DecodeConfig should have comment or be unexported
common/errors/errors.go:21:1: comment on exported var FeatureNotAvailableErr should be of the form "FeatureNotAvailableErr ..."
common/errors/errors.go:23:5: error var FeatureNotAvailableErr should have name of the form ErrFoo
common/maps/scratch.go:76:1: comment on exported method Scratch.Delete should be of the form "Delete ..."
common/maps/scratch.go:133:1: exported function NewScratch should have comment or be unexported
common/types/types.go:44:1: exported function NewKeyValuesStrings should have comment or be unexported
commands/hugo.go:65:1: exported method Response.IsUserError should have comment or be unexported
commands/import_jekyll.go💯21: error strings should not be capitalized or end with punctuation or a newline
commands/server.go:417:1: receiver name sc should be consistent with previous receiver name s for serverCmd
Doing so was probably a mistake. This may be a breaking change for some people,
but it's easy to restore the previous behaviour in the layouts.
Fixes#5172
