Convert Swift Differentiable Programming Implementation Overview to Markdown

[philipturner]

This PR entails adding new files to the "Documentation" directory.

Add the top of this Google Doc, there was a request to convert it to Markdown. I spent several hours doing that to let us resume work on those plans, using GitHub instead of Google Docs. For more context about this PR, check out this issue: #1185 (comment). I am making this a PR to main since I haven't heard back from anybody at the time of writing.

The text below appears at the top of the new Markdown file, and will be removed once all changes are verified. The added documentation will not be hyperlinked to by any other files within tensorflow/swift-apis yet, although the public will be able to see that new commits were recently made.

Some links required to navigate my changes are broken in this post. Please go to the Markdown file instead and click the links there.

Philip Turner: This note ensures changes aren't buried under a comment thread until they're individually approved. In addition, some section hyperlinks might break in a PR comment. Each line in this header will be deleted as it is approved.

After that is done, the document can be merged into tensorflow/swift-apis@main and hyperlinked in the Differentiable Programming Manifesto. The original Google Doc will receive a deprecation notice like the old Differentiable Programming Manifesto Draft. This action will commence the resurrection of S4TF.

• I went ahead and added myself to the list of authors/contributors. In addition, I added a table of contents and merged some otherwise line-separated text in Terminology to make it more legible in Markdown.

• Certain comments and instances of the 🚧 emoji were converted into TODO asides.

• The links are unchanged, with an exception. There is an outdated Google Docs draft that was transferred into the Differentiable Programming Manifesto. I mapped all of those links to appropriate sections in the new location.

• Explicit function conversion (in Differentiable function type conversion) no longer works as of Swift 5.5. May I put a warning there to alert readers?

• I added a note in the Registration section, warning that the custom differentiation tutorial is not up to date. When Swift supports online DocC, we can replace it. For more context - [1] [2] under the "Requesting assistance" issue.

• The highlighting for Activity analysis doesn't work in Markdown. I changed the highlighted syntax in that section to all caps.

I will edit my post to check off each task as it is approved. Simultaneously, they will be deleted from the document in my fork.

[philipturner]

@googlebot I signed it!

[philipturner]

Repository managers, please rescan this PR for the Google CLA.

[philipturner]

Closing PR to attempt a second time, hopefully with Google CLA working.