Only enable support files when there are related pages

[caendesilva]

Fixes #443

• The RSS feed related generators are now only enabled when there are blog posts
• The documentation search related generators are now only enabled when there are documentation pages

This may seem like a big change, but in actuality, these files were not generated when running the standard build command unless the pages existed, so it makes no sense to have links to them in the site.

Comment related to the issue:

Okay, so this was discovered when running php hyde build on a new project without any posts. An RSS feed is not generated then as the build command has an extra check to see if there are any posts. Same with documentation. This is of course inconsistent with how we handle it in other places. So something needs to be changed. It then begs the design question, should we add generate these files (rss and search) when there are no files for them to link to? I think no, but I could be wrong. For now, I think I will merge the extra checks in the build command to apply to the features check directly.

Originally posted by @caendesilva in #443 (comment)

[codecov]

Codecov Report

Merging #448 (9a31f05) into master (25de138) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #448   +/-   ##
=========================================
  Coverage     99.95%   99.95%           
  Complexity      914      914           
=========================================
  Files           106      106           
  Lines          2258     2258           
=========================================
  Hits           2257     2257           
  Misses            1        1           

Impacted Files	Coverage Δ
...mework/src/Commands/HydeBuildStaticSiteCommand.php	100.00% <100.00%> (ø)
packages/framework/src/Helpers/Features.php	100.00% <100.00%> (ø)

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