docs: add excerpts to blog posts

[vishalvivekm]

Description

Currently the blog post index page https://keptn.sh/stable/blog/ shows all of the blog posts with all of their content in one big page.

This PR

• creates posts excerpt using the mkdocs features highlighted in the issue's description.
• makes excerpt required in all the blogs pages.

Fixes #2923

Checks

• My PR fulfills the Definition of Done of the corresponding issue and not more (or parts if the issue is separated
  into multiple PRs)
• I used descriptive commit messages to help reviewers understand my thought process
• I signed off all my commits according to the Developer Certificate of Origin (DCO)(
  see Contribution Guide)
• My PR title is formatted according to the semantic PR conventions described in
  the Contribution Guide
• My content follows the style guidelines of this project (YAMLLint, markdown-lint)
• I regenerated the auto-generated docs for Helm and the CRD documentation (if applicable)
• I have performed a self-review of my content including grammar and typo errors and also checked the rendered page
  from the Netlify deploy preview
• My changes result in all-green PR checks (first-time contributors need to ask a maintainer to approve their test runs)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[vishalvivekm]

@mowies done.

Here's how blog page https://keptn--3008.org.readthedocs.build/3008/blog/ looks now:

[mowies]

LGTM!

[mowies]

LGTM!