Add demo doc pages, drop tmp git submodule

[chalin]

• Contributes to Migrate demo docs to this repo #2269
• Adds the demo doc pages "as is" (as they were created from the adjust script). I'll do clean up in subsequent PRs
• Drops the demo git submodule
• Reverts changes to the submodule-docs adjust and copy scripts

Preview: https://deploy-preview-2270--opentelemetry.netlify.app/docs/demo/

Note: I'll update the CODEOWNERS file in a followup PR.

/cc @open-telemetry/demo-approvers

---

Note: Other than page meta data (date modified, etc), and GH commit references, there are no changes to the generated site files. And only the demo doc pages (and the sitemap) are affected by this PR:

$ (cd public && git diff -bw --ignore-blank-lines) | grep ^diff | wc -l
      38
$ (cd public && git diff -bw --ignore-blank-lines) | grep ^diff | head 
diff --git a/docs/demo/current_architecture/index.html b/docs/demo/current_architecture/index.html
diff --git a/docs/demo/demo_features/index.html b/docs/demo/demo_features/index.html
diff --git a/docs/demo/demo_screenshots/index.html b/docs/demo/demo_screenshots/index.html
...
$ (cd public && git diff -bw --ignore-blank-lines) | grep ^diff | grep -v /docs/demo/
diff --git a/sitemap.xml b/sitemap.xml
$ (cd public && git diff -bw --ignore-blank-lines -I 'dateModified|lastmod|modified_time|text-muted|Last modified|/div|og:site_name|^\s+$') | head
$

[mviitane]

I'm missing the original folder structure in the navigation. Now, only one of the folders are visible in the preview. However, it's fine if the improved navigation is added in the following PRs.

[julianocosta89]

The file content/en/docs/demo/img/opentelemetry-logo-nav.png is not being used anymore.
We should be good to remove it.

[chalin]

@mviitane @julianocosta89: yes, I'll address those issues in followup PRs. Can one of you (or another demo-approver) approve this PR?

[chalin]

I'm missing the original folder structure in the navigation. Now, only one of the folders are visible in the preview. However, it's fine if the improved navigation is added in the following PRs.

Here's the followup issue:

• Demo docs are missing the original folder structure in the nav #2274

[chalin]

And here's the followup PR:

• Demo docs: left-nav fixes, adjusted linkTitles and more #2275