Docs: Convert document Overviews and Best Practices to markdown

[jordandrako]

Pull request checklist

• Merge PR ComponentPage: Add Edit On GitHub buttons to page sections #4424 before merging this one.
• Include a change request file using $ npm run change

Description of changes

For all components (excluding those in Experiments package)

• Add componentUrls to component example pages
• Move Overview to ./docs/Overview.md file
• Move BestPractices to ./docs/BestPractices.md file
• Move Dos to ./docs/Dos.md file
• Move Donts to ./docs/Donts.md file

Also changed/updated/fixed:

• Rewrote the Themes page to use ComponentPage pattern like all the other components. This enabled me to use markdown for the description.
  • Removed hardcoded palette values
  • Added palette and semanticColors values using getTheme(true) instead
  • See diff here https://github.com/OfficeDev/office-ui-fabric-react/pull/4512/files#diff-d8f8aa6abda1b50ed3efd732fa2d3110R2
• Made changes to some functions in theme.ts
  • Fixed a bug with _makeSemanticColorsFromPalette that returned empty strings for deprecrated semanticColors.
  • Added opt-in argument for getTheme, loadTheme, and createTheme functions to return /* @deprecated */ on deprecated semanticColors. This is mainly for documentation that calls the function (like the rewritten ThemePage). It's opt-in so it doesn't break existing parsers.
  • See diff here https://github.com/OfficeDev/office-ui-fabric-react/pull/4512/files#diff-6118735142300c79db4d510e148e28efR43

Let me know if the bugfix and opt-in, or any other added changes should be in separate PRs.

Focus areas to test

Test if edit button link will go to the right place.

[mikewheaton]

I think it makes sense to use the same code owners as the rest of the component for now, but over time we'll want to find design owners who can review the usage guidelines.

[manishgarg1]

[manishgarg1]

Approved with the note that this is just a move. It is hard to read through all comments. Are there any changes that are not just a physical move?

[jordandrako]

@manishgarg1 The only changes other than converting to markdown:

• Add componentUrl to each page to enable the edit buttons.
• Fixing any grammar or spelling mistakes I found in my quick scanning.
• Add link to our Icon document to Icon overview and IconButton example.

[jordandrako]

Overhauled the "Themes" page so I could use markdown. It now pulls the theme palettes dynamically instead of being hardcoded. Added semanticColors as well.

[jordandrako]

Themes page before:

Themes page after:

[phkuo]

@jordandrako does it leave out the deprecated semantic slots?

[jordandrako]

@phkuo Finished adding @deprecated opt-in functionality and now the documentation shows deprecated color slots as deprecated.

[jordandrako]

@Jahnp @dzearing Do you have any issues with the changes here and can we get this wrapped up? See my PR comment for the full list of changes.

[Jahnp]

Sorry for the delay @jordandrako. This is an awesome set of changes. Looking now.