techdocs: Remove spinners

Spinners cause unnecessary UX distraction.
Case 1 (when docs are built and are to be served): Spinners appear for a split second before the name of site shows up. This unnecessarily distracts eyes because spinners increase the size of the Header. A dot (.) would do fine. Definitely more can be done.
Case 2 (when docs are being generated): There is already a linear progress bar (which is recommended in Storybook). Spinners are not good. 🤷

plugins/techdocs/src/reader/components/TechDocsPageHeader.tsx

@@ -16,7 +16,6 @@
 
 import React from 'react';
 import { AsyncState } from 'react-use/lib/useAsync';
-import { CircularProgress } from '@material-ui/core';
 import CodeIcon from '@material-ui/icons/Code';
 import { EntityName } from '@backstage/catalog-model';
 import { Header, HeaderLabel, Link } from '@backstage/core';
@@ -86,7 +85,7 @@ export const TechDocsPageHeader = ({
 
   return (
     <Header
-      title={siteName ? siteName : <CircularProgress />}
+      title={siteName ? siteName : '.'}
       pageTitleOverride={siteName || name}
       subtitle={
         siteDescription && siteDescription !== 'None' ? siteDescription : ''