Fix broken meta tags (description, og:title, og:description)

To review, you can use the system i describe below or build the PR locally and then inspect the headers. If you are going for the latter, you'll see that now the fields description, og:title and og:description don't show a placeholder but an actual value.

To make reviews easier, i recreated this branch on gitlab and deployed it on gitlab pages. This allows to test the meta tags directly on a deployed updated version of getmonero which runs this PR: https://erciccione.gitlab.io/monero-site-deploy (https://gitlab.com/erciccione/monero-site-deploy)

Be aware that the CSS is broken and i didn't bother to investigate the reason, since we only need it for the meta tags. You can just copy-paste the address of whatever page you want to test or take an url from Getmonero, substitute the first part of the url (https://getmonero.org) with https://erciccione.gitlab.io/monero-site-deploy and paste it in an online meta tag checker. I used Heymeta.com and opengraphcheck.com.

For example, to test the moneropedia entry Account:

1. navigate to getmonero until you find the URL, which in this case is https://web.getmonero.org/resources/moneropedia/account.html
2. Substitute the first part of the ulr with the updated version of getmonero on gitlab and you get: https://erciccione.gitlab.io/monero-site-deploy/resources/moneropedia/account.html
3. copy this URL and paste it in one of the websites mentioned above and you'll see the meta tags working properly: https://opengraphcheck.com/result.php?url=https%3A%2F%2Ferciccione.gitlab.io%2Fmonero-site-deploy%2Fresources%2Fmoneropedia%2Faccount.html
   https://www.heymeta.com/url/erciccione.gitlab.io/monero-site-deploy/resources/moneropedia/account.html

This process if the same for every page.

This PR fixes #925 (closed) and introduces working meta tags, which will improve the SEO of the website. The system is still quite minimal, but sets the basis for future improvements, like more capillar description tags.