[Docs] Add developer documentation for using/modifying the chrome service

src/core/public/chrome/README.md

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+
+##    Chrome Service
+
+- [About!](#about-)
+- [Nav Controls Service](#navcontrolsservice-)
+- [Nav Links Service](#navlinksservice-)
+- [Recently Accessed Service](#recentlyaccessedservice-)
+- [Doc Title Service](#doctitleservice-)
+- [UI](#ui-)
+
+## About :
+- **Signature** - `export interface ChromeStart`
+The chrome service is a high level UI service that is part of CoreStart (Core services exposed to the Plugin start lifecycle) and offers other plugins a way to add navigation controls to the UI, edit the document title, manipuate navlinks on global header as well as edit the recent accessed tab. It consists of these sub-services/components.
+
+- NavControlsService : for registering new controls to be displayed in the navigation bar.
+- NavLinksService : for manipulating nav links.
+- RecentlyAccessedService : for recently accessed history.
+- DocTitleService: for accessing and updating the document title
+- UI : All the UI components,icons e.g.  header, loaders.
+
+
+- How to access ? add in interface `chrome: ChromeStart && chrome.servicesName => e.g chrome.docTitle.method`
+- Where it is getting Registered/Executed ? [Staring Point](https://github.com/opensearch-project/OpenSearch-Dashboards/blob/2.1/src/core/public/rendering/rendering_service.tsx)
+- How header component is getting rendered ? `const chromeUi = chrome.getHeaderComponent(); `
+- Chrome Methods </b> [See chrome interface/methods](https://github.com/opensearch-project/OpenSearch-Dashboards/blob/2.1/src/core/public/chrome/chrome_service.tsx)
+
+
+### NavControlsService : 
+- Interface : ChromeNavControls
+- **Signature** - `navControls: ChromeNavControls`
+- Registering new controller 
+
+Example : 
+Register a left-side nav control rendered with React.
+
+```jsx
+chrome.navControls.registerLeft({
+  mount(targetDomElement) {
+    ReactDOM.mount(<MyControl />, targetDomElement);
+    return () => ReactDOM.unmountComponentAtNode(targetDomElement);
+  }
+})
+```
+### NavLinksService : 
+- Interface :  ChromeNavLinks
+- **Signature** - `navLinks: ChromeNavLinks`
+- e.g. Method : 
+
+Get an observable for a sorted list of navlinks :-
+
+`getNavLinks$(): Observable<Array<Readonly<ChromeNavLink>>>`
+
+Get the state of a navlink at this point in time :-
+
+`get(id: string): ChromeNavLink | undefined`
+
+Get the current state of all navlinks :-
+
+`getAll(): Array<Readonly<ChromeNavLink>>`
+
+Check whether or not a navlink exists :-
+
+`has(id: string): boolean`
+
+Remove all navlinks except the one matching the given id :-
+`showOnly(id: string): void`
+
+- How to access
+  ###### Get the current state of all navlinks: 
+  `core.chrome.navLinks.getAll()`
+
+  ###### Get the state of a navlink at this point in time: 
+  `core.chrome.navLinks.get()`
+
+### RecentlyAccessedService : 
+
+- Interface : ChromeRecentlyAccessed
+- Signature : ```recentlyAccessed: ChromeRecentlyAccessed```
+- The Recently viewed items are stored in the browser's local storage.
+- You can go back to the recent search/visualization/dashboard , each item has (link,label,id)
+- Methods :
+
+Adds a new item to the recently accessed history :-
+
+`add(link: string, label: string, id: string): void`
+
+Gets an Array of the current recently accessed history :-
+
+`get(): ChromeRecentlyAccessedHistoryItem[]`
+
+Gets an Observable of the array of recently accessed history :-
+
+`get$(): Observable<ChromeRecentlyAccessedHistoryItem[]>`
+
+- How to access
+   ###### Adds a new item to the recently accessed history :
+   `
+   chrome.recentlyAccessed.add('/app/map/1234', 'Map 1234', '1234');
+   `
+   ###### Gets an Array of the current recently accessed history :
+   `
+    chrome.recentlyAccessed.get().forEach(console.log);;
+   `
+
+### DocTitleService : 
+- Interface : ChromeDocTitle
+- **Signature** - `docTitle: ChromeDocTitle`
+  ###### - How to change the title of the document
+
+
+   ```ts
+   chrome.docTitle.change('My application title')
+   chrome.docTitle.change(['My application', 'My section'])
+   ```
+### UI :
+###### consists of tsx/scss files && renders UI components from css Library e.g ```<Progress props={props}>```
+
+###### Adding/overriding existing css : 
+- Create scss file and define class e.g .osdCustomClass{}
+- pass className prop to UI component.
+        e.g `<HeaderBreadcrumbs className="osdCustomClass"/> `
+
+###### UI Components :
+  - HeaderBreadcrumbs : Props
+
+  -  responsive:boolean Hides extra (above the max) breadcrumbs under a collapsed item as the window gets smaller.
+  -  max: Collapses the inner items past the maximum set here into a single ellipses item.      
+  -  breadcrumbs : The array of individual Breadcrumb items
+
+