Many spelling and code readability improvements.

[antaldaniel]

I would like to make some more meaningful commits, but I found the documentation and the code hard to read, so I thought first I would make a pull request with very obvious changes. None of the changes modify any code.

• Plenty of spelling mistakes are corrected.
• Because the DESCRIPTION sets the spelling to en-US, en-GB spelling was modified to en-US (when multiple uses were present.)
• Some missing Roxygen fields (such as missing @return) values are inserted. In the next CRAN release, this would have triggered a review.
• When functions were referenced from another package, such as httr::POST, the manual was changed to \code{httr:\link[httr::POST]{POST}} so that manuals are linked per CRAN policy.
• In some cases, I added concise extra URL links or details, particularly with abbreviations that may put off a first-time user; for example, I linked or entirely wrote out QID as Wikidata Q ID (unique ID in Wikidata), linking the official definition.
• Also linked the Wikidata Query Statement Manual with URL.
• When the code or documentation lines exceeded 100 characters, I added extra indentation for code readability.
• When Wikidata was referenced in the text, it followed either the official, capitalized Wikidata spelling or wikidata, I made it Wikidata in the documentation (of course, not in the code and URLs.)

There are a few non-obvious changes that I did not make without asking.

• It would make sense to use usethis::use_spell_check to avoid piling up again hundreds of typos.
• The README is already quite long, but some expressions are not explained to a new user of Wikidata. I think it would make sense to re-write it with extra detail into 2-3 vignette articles, largely following with a more detailed explanation of the current code divisions, for example, writing a vignette with examples on how to find things on Wikidata programmatically with the Package.
• There are some good coding practices and intentendation rules that would make the code easier to read for a new contributor and improve without changing code functionality.
• It would be good to add a pkgdown website and install a long-form, standard, HTML manual which is far easier to read than the PDF manual.

[hugolpz]

Hello @TS404 @Ironholds @metacoretechs,

Could you review this PR and, if necessary, grand adminship to @cutterkom . As WikidataR move to open source model and reduced coding intensity, it is important to multiply github administrators so the project stays resilient and reactive to PR.

Also, thank you for this library, I will recommend a group of PhD student its use tomorrow, in a seminar.