Use consistent naming in this repository -- plus codes or open location code

[fulldecent]

The README and FAQ here clarify:

• That "plus codes" (always lower case, similar to "postal codes") are the things that baseline humans and postpeople are going to use to find things on this big blue/green orb we live on; and
• That "Open Location Code" (always singular, always upper case, a proper noun) is the technology that developers will use to create the plus codes.

This messaging is frustrated by several inconsistencies in this repository.

Work plan

• Change every reference to "Open Location Codes"
  • These words literally mean "forks of the Open Location Code repository". Instead those should be referring to probably "plus codes" and or "Open Location Code".
• Rename all implementations to "Open Location Code".
  • Implementations here are named "PlusCodes" "Plus+Codes" "Plus Codes" and more. These violate the naming established above and contribute to confusion.
  • Every implementation Google publishes (i.e. in this repository) should be named just "Open Location Code".
  • It is not established that "Open Location Code" is a name that is dedicated to the public domain. i.e. I'm not sure if it is appropriate for me to write an Open Location Code implementation and then name that implementation, for example, "Solidity Open Location Code". This could be a separate issue.
• Review every remaining mention of /Open Location Codes?/i and /plus.{0,5}codes/I in this repo to find stray word misusage.

Work plan outside this project (e.g. advocacy)

• Reach out to every project/repository that is linked from this project, identify any word misusage, and recommend to fix it

[fulldecent]

Separately, this is mentioned in https://github.com/google/open-location-code/wiki/Naming-guidelines and that needs to be brought into one of the normative specifications

[fulldecent]

Recommended tags: implementation specification documentation

[fulldecent]

This is partially fixed at #463

The documentation is updated to make consistent and correct usage of Plus Code versus Open Location Code.

Implementations are not updated. I hope this PR can please be reviewed and merged before I consider the implementations.