[FLINK-21369][docs] Document Checkpoint Storage

[sjwiesman]

What is the purpose of the change

Add documentation for the new Checkpoint Storage abstraction and new State Backend apis (HashMap and EmbeddedRocksDB).

I also cleaned up a few broken links and other misc I found along the way

[sjwiesman]

cc @rkhachatryan & @alpinegizmo

[flinkbot]

Thanks a lot for your contribution to the Apache Flink project. I'm the @flinkbot. I help the community
to review your pull request. We will use this comment to track the progress of the review.

Automated Checks

Last check on commit 95e05ce (Fri Feb 12 19:46:25 UTC 2021)

✅no warnings

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

Please see the Pull Request Review Guide for a full explanation of the review process.

The Bot is tracking the review progress through labels. Labels are applied according to the order of the review items. For consensus, approval by a Flink committer of PMC member is required

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot approve description to approve one or more aspects (aspects: description, consensus, architecture and quality)
• @flinkbot approve all to approve all aspects
• @flinkbot approve-until architecture to approve everything until architecture
• @flinkbot attention @username1 [@username2 ..] to require somebody's attention
• @flinkbot disapprove architecture to remove an approval you gave earlier

[flinkbot]

CI report:

• 030c2d5 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run travis re-run the last Travis build
• @flinkbot run azure re-run the last Azure build

[alpinegizmo]

Basically looks good, but I found a bunch of nits and had a question or two whether something is correct.

[sjwiesman]

@alpinegizmo thank you for the review, I hope I've addressed all your comments.

Sorry for force pushing, I messed up a rebase.

[sjwiesman]

mergind