Move buffering info to the "Data pipeline" section

[alexakreizinger]

this PR:

• moves the "Buffering" doc (not the "Buffering and storage" doc, which is a different thing) from the "Concepts" section to the "Data pipeline" section
• updates some links associated with that move
• renames "Router" to "Routing" for consistency
• adds a mermaid diagram to the "Concepts > Data pipeline" page
• rearranges the sections in "Concepts > Data pipeline" (particularly because the intro says "these things happen in this order" but then lists them in an order that does not match how they actually happen)

[cosmo0920]

I rechecked the code of the sequence of pipeline construction on Fluent Bit.
And I realized and confirmed it's correct descriptions.
And adding diagram is great work! 👍