Corrections and additions in devdocs (mainly php structure)

Closes none

What does this MR do?

Collection of small changes in the devdocs that I notice during doing other things.

• php structure as discussed in a call with @janopae & @NerdyProjects
• removing whitespace at line endings
• mentioning the GUI for REST API documentation
• fix a few broken links

Can be merged at any time.

How confident are you it won't break things if deployed?

Sure. Only devdoc changes. Compiling locally works as expected.

Links to related issues

Might overlap a bit with !1464 (merged) Requires !1475 (merged) because beforehand it's not correct.

How to test

See https://devdocs.foodsharing.network/about-devdocs.html#setting-things-up and then http://localhost:4001/database.html#database-migration

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• not necessary: Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings