Improve documentation regarding WP API REST urls about docs

[mburridge]

Add a note to check permalink settings in 'Initial checks' section: https://docs.frontity.org/getting-started#wordpress

[mburridge]

Added

Check also that you are using one of the pretty permalinks options, rather than the plain one, in Settings->Permalinks.

to docs/getting-started/README.md.

Added

In particular we would like to draw your attention to the Initial checks section.

to docs/README.md

[juanmaguitar]

@mburridge Let's try to add links to the specific commits/PR's/issues when we adress any issue

In this case
fbb3bc9
for example

Also managing this with a PR would be better.
Even if you directly merge it because there's no need to review it (but in this case I'd have liked to comment a few things)

Let's take this into account for the next one 😉