SEO optimization for the Documentation website

[evnchn]

Motivation

SEO's important but we really did not do a very good job of it in the documentation website.

#4804 (reply in thread)

Implementation

1. Preliminary attempt at adding title and keywords
2. Due to lack of time, infer description from any sources I can get my hands on.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

Consider using https://github.com/Flozad/onpage-seo-chrome-extension for veryifying stuff works in a preliminary level. You need more powerful SEO tools to do anything more than that.