Add CommonMark as optional text format suggestion to instance description

[6543]

As per #78 (comment)

add suggestion to have text formatted in CommonMark.

[6543]

examples already using formation:

• sloth.run: CommonMark
• soshar.dess.ga: HTML
• darnell.day: CommonMark
• kira.zone: HTML
• social.shiggy.fun: HTML

data from the-federation.info

select count(node.domain) from (select node_id,data->'metadata'->'nodeDescription'::text as d from node_info where data->'metadata'->'nodeDescription' IS NOT NULL) JOIN node on node.id=node_id WHERE d::text like '%<br>%';

-> 263

select count(node.domain) from (select node_id,data->'metadata'->'nodeDescription'::text as d from node_info where data->'metadata'->'nodeDescription' IS NOT NULL) JOIN node on node.id=node_id WHERE d::text like '%](%';

-> 57

so you could say most use HTML but CommonMark is also used.