Add copy to Circumvention Tool Charts page

[agrabeli]

We recently did a soft launch of a new Circumvention Tool Chart page on OONI Explorer: https://explorer.ooni.org/chart/circumvention

However, I think 2 things are missing:

1) Brief text explaining what this page is, linking to the relevant test methodologies

2) Links to Tor Metrics & the Psiphon Data Engine (which would enable users to gain a more holistic view of the usage of these tools)

I therefore propose adding a paragraph at the start of the page (perhaps under the title) with the following text:

The charts below display aggregate views of OONI data based on the testing of the following circumvention tools:

* [Psiphon](https://ooni.org/nettest/psiphon)

* [Tor](https://ooni.org/nettest/tor)

* [Tor Snowflake](https://ooni.org/nettest/tor-snowflake/)

Please note that the presence of [anomalous measurements](https://ooni.org/support/faq/#what-do-you-mean-by-anomalies) is not always indicative of blocking, as [false positives](https://ooni.org/support/faq/#what-are-false-positives) can occur. Moreover, circumvention tools often have built-in circumvention techniques for evading censorship. 

We therefore recommend referring to **[Tor Metrics](https://metrics.torproject.org/)** and to the **[Psiphon Data Engine](https://psix.ca/)** to view usage stats and gain a more comprehensive understanding of whether these tools work in each country. 

[iArchitSharma]

@hellais Hi can I work on this issue

[hellais]

This is done