Add "related datasets" for the top 10 datasets

As a Datahub team, we want to promote similar datasets on our popular showcase pages so that users can find exactly what they're looking for.

Acceptance criteria

[x] our most popular showcase pages have 'Similar datasets' section with the links to related showcase pages

Selected showcase pages and related datasets:

s-and-p-500-companies
- s-and-p-500
- s-and-p-500-companies-financials
- finance-vix
- nyse-other-listings
language-codes
- country-list
- country-codes
- continent-codes
- un-locode
s-and-p-500
- s-and-p-500-companies
- s-and-p-500-companies-financials
- finance-vix
- nyse-other-listings
world-cities
- country-list
- country-codes
- population
- continent-codes
continent-codes
- country-list
- language-codes
- airport-codes
global-temp
- co2-ppm
- co2-fossil-by-nation
- sea-level-rise
- glacier-mass-balance
un-locode
- country-list
- country-codes
- airport-codes
- language-codes
country-codes
- country-list
- language-codes
- airport-codes
- continent-codes
gold-prices
- oil-prices
- natural-gas