oppia: [BUG]: Revision cards (and some other pages) are not getting picked up by search engines

Describe the bug

The Oppia revision cards (and probably other pages on Oppia) are not getting indexed by search engines, which means that learners aren’t able to find them. We should figure out why this is happening and fix it (e.g. perhaps the Revision tab in the topic page is not being recognized by search engines as a crawlable link, and/or we need to update our sitemap).

Steps To Reproduce

1. Visit any revision card on Oppia, such as https://www.oppia.org/learn/math/multiplication/revision/rules-to-simplify (you can also get to the full suite of lessons by going to https://www.oppia.org/learn/math )
2. Try searching for that card in Google / Bing / etc. search, using the query "name of revision card" oppia. For example: "commutative property of multiplication" oppia or "rules to simplify multiplication" oppia.
3. Observe that there are no search results pointing to that revision card.

Expected Behavior

The Oppia revision card should show up in search engine results, especially for queries that include the title or headings of that card.

Similar behavior should happen for other pages on Oppia as well – all pages should be accessible from search engines.

Tips for developers

If this is your first issue, please make sure to follow https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#choosing-a-good-first-issue and https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#setting-things-up before claiming it. Thanks!