Add new return-values and key-values examples - 2.1

[lparisc]

Backport of PR #450 to 2.1

Resolves devrel-1524

Add new stable releases of the following examples:

• https://github.com/EOSIO/return-values-example-app
• https://github.com/EOSIO/key-value-example-app

To the manuals directory in dev portal under Examples:

• https://developers.eos.io/welcome/latest/manuals/index