Publish a one page doc about supported data source spec

[shawncao]

This is a topic that we haven't had a post of doc page talking about, and I think it's the most matter thing to users who want to adopt Nebula.

Similar to https://nebula.bz/sdk.html, let's add a doc page for "ingestion" and link to the menu bar in the home page at https://nebula.bz, also we can have this ingestion page linked in repo home README.md as it's something new users care about.

[shawncao]

@chenqin thought you might be a good candidate to own this? are you willing to take this? if yes, assign to yourself or I can assign it to you. We should document all supported data source types as well as the time spec, supported macro etc.

[chenqin]

drafting google doc

[chenqin]

remove serde.retention use retention.max-hr instead
#56