Getting started guide fixes & Explain X-SendFile a little better, with links

Explain how form_for :article is able to pull in the properties of @article
Make it clear that article_id is generated due to the association set up

Add link to the rails function that uses X-Sendfile.
Add links to apache and nginx docs for the header
Revert "Fixed plugin_generator test"

This reverts commit fefa8ae9a172835fb6b8aef7d1dd46d58eecd49f.
Merge pull request #14069 from arunagw/aa-plugin-generator-build-fix

Revert "Fixed plugin_generator test"

As this commit  22a1a5ac8c4631f29cfeac451c361e8da1dd2261 has reverted all changes so this plugin generator also needs to fix.
Add a missing changelog entry for #13981 and #14035

[ci skip]
Merge pull request #14072 from robin850/patch-16

Add a missing changelog entry for #13981 and #14035
Merge pull request #13937 from ktaragorn/guides_fixes

Guides fixes