Consolidate stream processing pages

[alexakreizinger]

this PR:

• consolidates "intro to stream processing" and "get started" into the stream processing "overview" page
• removes a changelog that hasn't been updated in six years
• removes a page that wasn't being used at all
• moves the contents of "get started" up a level (+ renames one of the pages)
• adds redirects, swaps out links, and updates SUMMARY.md accordingly
• removes an image that was included in the deleted pages
• fixes a random typo that I just happened to catch when I accidentally did a ctrl+f on steam instead of stream :P

[patrick-stephens]

We really should have some CI to pick up broken links...

[esmerel]

We really should have some CI to pick up broken links...

I added a github action for this a couple of months back, but I only have it run once a month to catch broken links. It also catches a bunch of things that are valid but are blocked from bot-crawlers etc. I don't want all those false results popping up on every PR and I don't know how to restrict it to the files in a single PR so not running it on pull request is safer for now.

https://github.com/fluent/fluent-bit-docs/blob/master/.github/workflows/linkcheck.yaml